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\documentclass{article}
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Christian Urban <christian dot urban at kcl dot ac dot uk>
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\usepackage{hyperref}
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Christian Urban <christian dot urban at kcl dot ac dot uk>
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\usepackage{amssymb}
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Christian Urban <christian dot urban at kcl dot ac dot uk>
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\usepackage{alltt}
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Christian Urban <christian dot urban at kcl dot ac dot uk>
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\usepackage{menukeys}
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Christian Urban <christian dot urban at kcl dot ac dot uk>
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\usepackage{amsmath}
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Christian Urban <christian dot urban at kcl dot ac dot uk>
parents:
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\usepackage{../langs}
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Christian Urban <christian dot urban at kcl dot ac dot uk>
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\usepackage{mathpazo}
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Christian Urban <christian dot urban at kcl dot ac dot uk>
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\usepackage{marvosym}
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Christian Urban <christian dot urban at kcl dot ac dot uk>
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%%%\usepackage[scaled=.95]{helvet}
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Christian Urban <christian dot urban at kcl dot ac dot uk>
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Christian Urban <christian dot urban at kcl dot ac dot uk>
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\newcommand{\dn}{\stackrel{\mbox{\scriptsize def}}{=}}%
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Christian Urban <christian dot urban at kcl dot ac dot uk>
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\definecolor{codegray}{gray}{0.9}
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Christian Urban <christian dot urban at kcl dot ac dot uk>
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\newcommand{\code}[1]{\colorbox{codegray}{\texttt{#1}}}
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Christian Urban <christian dot urban at kcl dot ac dot uk>
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Christian Urban <christian dot urban at kcl dot ac dot uk>
parents:
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\begin{document}
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Christian Urban <christian dot urban at kcl dot ac dot uk>
parents:
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|
17 |
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Christian Urban <christian dot urban at kcl dot ac dot uk>
parents:
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|
18 |
\section*{A Crash-Course on Scala}
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Christian Urban <christian dot urban at kcl dot ac dot uk>
parents:
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230
Christian Urban <christian dot urban at kcl dot ac dot uk>
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|
20 |
Scala is a programming language that combines functional and
|
228
Christian Urban <christian dot urban at kcl dot ac dot uk>
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|
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object-oriented programming-styles, and has received in the
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232
Christian Urban <christian dot urban at kcl dot ac dot uk>
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|
22 |
last five years or so quite a bit of attention. One reason for
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
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|
23 |
this attention is that, like the Java programming language,
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
24 |
Scala compiles to the Java Virtual Machine (JVM) and therefore
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
25 |
Scala programs can run under MacOSX, Linux and
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
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|
26 |
Windows.\footnote{There are also experimental backends for
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
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|
27 |
Android and JavaScript.} Unlike Java, however, Scala often
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
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|
28 |
allows programmers to write very concise and elegant code.
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
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|
29 |
Some therefore say Scala is the much better Java. Some
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
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|
30 |
companies (The Guardian, Twitter, Coursera, LinkedIn to name a
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
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|
31 |
few) either use Scala excusively in production code, or some
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
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|
32 |
part of it are written in Scala. If you want to try out Scala
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
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|
33 |
yourself, the Scala compiler can be downloaded from
|
227
Christian Urban <christian dot urban at kcl dot ac dot uk>
parents:
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|
34 |
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Christian Urban <christian dot urban at kcl dot ac dot uk>
parents:
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|
35 |
\begin{quote}
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
parents:
diff
changeset
|
36 |
\url{http://www.scala-lang.org}
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
parents:
diff
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|
37 |
\end{quote}
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
parents:
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|
38 |
|
230
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
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|
39 |
Why do I use Scala in the AFL module? Actually, you can do
|
228
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
40 |
\emph{any} part of the programming coursework in \emph{any}
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
41 |
programming language you like. I use Scala for showing you
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
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|
42 |
code during the lectures because its functional
|
229
Christian Urban <christian dot urban at kcl dot ac dot uk>
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|
43 |
programming-style allows me to implement the functions we will
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
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|
44 |
discuss with very small code-snippets. Since the compiler is
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
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|
45 |
free, you can download them and run every example I give. But
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
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|
46 |
if you prefer, you can also easily translate the code-snippets
|
230
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
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|
47 |
into any other functional language, for example Haskell,
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
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|
48 |
Standard ML, F\#, Ocaml and so on.
|
227
Christian Urban <christian dot urban at kcl dot ac dot uk>
parents:
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|
49 |
|
230
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
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|
50 |
Developing programs in Scala can be done with the Eclipse IDE
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
51 |
and also with IntelliJ IDE, but for the small programs I will
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
52 |
develop the good old Emacs-editor is adequate for me and I
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
53 |
will run the programs on the command line. One advantage of
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
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|
54 |
Scala over Java is that it includes an interpreter (a REPL, or
|
229
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
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|
55 |
Read-Eval-Print-Loop) with which you can run and test small
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
56 |
code-snippets without the need of the compiler. This helps a
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
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|
57 |
lot with interactively developing programs. Once you installed
|
232
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
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|
58 |
Scala correctly, you can start the interpreter by typing on
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
59 |
the command line:
|
228
Christian Urban <christian dot urban at kcl dot ac dot uk>
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|
60 |
|
230
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
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|
61 |
\begin{quote}
|
227
Christian Urban <christian dot urban at kcl dot ac dot uk>
parents:
diff
changeset
|
62 |
\begin{alltt}
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
parents:
diff
changeset
|
63 |
$ scala\small
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
parents:
diff
changeset
|
64 |
Welcome to Scala version 2.11.2 (Java HotSpot(TM) 64-Bit Server VM).
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
parents:
diff
changeset
|
65 |
Type in expressions to have them evaluated.
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
parents:
diff
changeset
|
66 |
Type :help for more information.\normalsize
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
parents:
diff
changeset
|
67 |
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
parents:
diff
changeset
|
68 |
scala>
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
parents:
diff
changeset
|
69 |
\end{alltt}
|
230
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
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|
70 |
\end{quote}
|
227
Christian Urban <christian dot urban at kcl dot ac dot uk>
parents:
diff
changeset
|
71 |
|
229
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
72 |
\noindent The precise response may vary due to the platform
|
230
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
73 |
where you installed Scala. At the Scala prompt you can type
|
228
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
74 |
things like {\tt 2 + 3} \keys{Ret} and the output will be
|
227
Christian Urban <christian dot urban at kcl dot ac dot uk>
parents:
diff
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|
75 |
|
230
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
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|
76 |
\begin{quote}
|
227
Christian Urban <christian dot urban at kcl dot ac dot uk>
parents:
diff
changeset
|
77 |
\begin{alltt}
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
parents:
diff
changeset
|
78 |
scala> 2 + 3
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
parents:
diff
changeset
|
79 |
res0: Int = 5
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
parents:
diff
changeset
|
80 |
\end{alltt}
|
230
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
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|
81 |
\end{quote}
|
227
Christian Urban <christian dot urban at kcl dot ac dot uk>
parents:
diff
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|
82 |
|
228
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
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|
83 |
\noindent indicating that the result of the addition is of
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
84 |
type {\tt Int} and the actual result is {\tt 5}. Another
|
229
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
85 |
classic example you can try out is
|
227
Christian Urban <christian dot urban at kcl dot ac dot uk>
parents:
diff
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|
86 |
|
230
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
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|
87 |
\begin{quote}
|
227
Christian Urban <christian dot urban at kcl dot ac dot uk>
parents:
diff
changeset
|
88 |
\begin{alltt}
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
parents:
diff
changeset
|
89 |
scala> print ("hello world")
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
parents:
diff
changeset
|
90 |
hello world
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
parents:
diff
changeset
|
91 |
\end{alltt}
|
230
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
92 |
\end{quote}
|
227
Christian Urban <christian dot urban at kcl dot ac dot uk>
parents:
diff
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|
93 |
|
230
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
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|
94 |
\noindent Note that in this case there is no result. The
|
229
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
95 |
reason is that {\tt print} does not actually produce a result
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
96 |
(there is no {\tt resXX}), rather it is a function that causes
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
97 |
the \emph{side-effect} of printing out a string. Once you are
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
98 |
more familiar with the functional programming-style, you will
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
99 |
know what the difference is between a function that returns a
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
100 |
result, like addition, and a function that causes a
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
101 |
side-effect, like {\tt print}. We shall come back to this
|
230
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
102 |
point later, but if you are curious now, the latter kind of
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
103 |
functions always have as return type {\tt Unit}.
|
227
Christian Urban <christian dot urban at kcl dot ac dot uk>
parents:
diff
changeset
|
104 |
|
230
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
105 |
If you want to write a stand-alone app in Scala, you can
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
106 |
implement an object that is an instance of {\tt App}, say
|
229
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
107 |
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
108 |
\begin{quote}
|
230
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
109 |
\begin{lstlisting}[language=Scala,numbers=none]
|
229
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
110 |
object Hello extends App {
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
111 |
println ("hello world")
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
112 |
}
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
113 |
\end{lstlisting}
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
114 |
\end{quote}
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
115 |
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
116 |
\noindent save it in a file, say {\tt hellow-world.scala}, and
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
117 |
then run the compiler and runtime environment:
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
118 |
|
230
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
119 |
\begin{quote}
|
229
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
120 |
\begin{alltt}
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
121 |
$ scalac hello-world.scala
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
122 |
$ scala Hello
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
123 |
hello world
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
124 |
\end{alltt}
|
230
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
125 |
\end{quote}
|
229
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
126 |
|
230
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
127 |
As mentioned above, Scala targets the JVM and consequently
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
128 |
Scala programs can also be executed by the bog-standard Java
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
129 |
Runtime. This only requires the inclusion of {\tt
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
130 |
scala-library.jar}, which on my computer can be done as
|
229
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
131 |
follows:
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
132 |
|
230
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
133 |
\begin{quote}
|
229
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
134 |
\begin{alltt}
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
135 |
$ scalac hello-world.scala
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
136 |
$ java -cp /usr/local/src/scala/lib/scala-library.jar:. Hello
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
137 |
hello world
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
138 |
\end{alltt}
|
230
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
139 |
\end{quote}
|
229
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
140 |
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
141 |
|
227
Christian Urban <christian dot urban at kcl dot ac dot uk>
parents:
diff
changeset
|
142 |
\subsection*{Inductive Datatypes}
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
parents:
diff
changeset
|
143 |
|
229
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
144 |
The elegance and conciseness of Scala programs are often a
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
145 |
result of inductive datatypes that can be easily defined. For
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
146 |
example in ``every-day mathematics'' we would define regular
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
147 |
expressions simply by giving the grammar
|
227
Christian Urban <christian dot urban at kcl dot ac dot uk>
parents:
diff
changeset
|
148 |
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
parents:
diff
changeset
|
149 |
\begin{center}
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
parents:
diff
changeset
|
150 |
\begin{tabular}{r@{\hspace{2mm}}r@{\hspace{2mm}}l@{\hspace{13mm}}l}
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
parents:
diff
changeset
|
151 |
$r$ & $::=$ & $\varnothing$ & null\\
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
parents:
diff
changeset
|
152 |
& $\mid$ & $\epsilon$ & empty string\\
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
parents:
diff
changeset
|
153 |
& $\mid$ & $c$ & single character\\
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
parents:
diff
changeset
|
154 |
& $\mid$ & $r_1 \cdot r_2$ & sequence\\
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
parents:
diff
changeset
|
155 |
& $\mid$ & $r_1 + r_2$ & alternative / choice\\
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
parents:
diff
changeset
|
156 |
& $\mid$ & $r^*$ & star (zero or more)\\
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
parents:
diff
changeset
|
157 |
\end{tabular}
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
parents:
diff
changeset
|
158 |
\end{center}
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
parents:
diff
changeset
|
159 |
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
parents:
diff
changeset
|
160 |
\noindent This grammar specifies what regular expressions are
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
parents:
diff
changeset
|
161 |
(essentially a kind of tree-structure with three kinds of
|
229
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
162 |
inner nodes---sequence, alternative and star---and three kinds
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
163 |
of leave nodes---null, empty and character). If you are
|
228
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
164 |
familiar with Java, it might be an instructive exercise to
|
230
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
165 |
define this kind of inductive datatypes in Java\footnote{Happy
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
166 |
programming! \Smiley} and then compare it how
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
167 |
it can be defined in Scala.
|
227
Christian Urban <christian dot urban at kcl dot ac dot uk>
parents:
diff
changeset
|
168 |
|
228
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
169 |
Implementing the regular expressions from above in Scala is
|
230
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
170 |
actually very simple: It first requires an \emph{abstract
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
171 |
class}, say, {\tt Rexp}. This will act as the type for regular
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
172 |
expressions. Second, it requires a case for each clause in the
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
173 |
grammar. The cases for $\varnothing$ and $\epsilon$ do not
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
174 |
have any arguments, while in all the other cases we do have
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arguments. For example the character regular expression needs
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
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|
176 |
to take as an argument the character it is supposed to
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
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|
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recognise. In Scala, the cases without arguments are called
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
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|
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\emph{case objects}, while the ones with arguments are
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
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|
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\emph{case classes}. The corresponding Scala code is as
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
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|
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follows:
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Christian Urban <christian dot urban at kcl dot ac dot uk>
parents:
diff
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|
181 |
|
228
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
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|
182 |
\begin{quote}
|
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Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
183 |
\begin{lstlisting}[language=Scala,numbers=none]
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Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
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|
184 |
abstract class Rexp
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
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|
185 |
case object NULL extends Rexp
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
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|
186 |
case object EMPTY extends Rexp
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
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|
187 |
case class CHAR (c: Char) extends Rexp
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
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|
188 |
case class SEQ (r1: Rexp, r2: Rexp) extends Rexp
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
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|
189 |
case class ALT (r1: Rexp, r2: Rexp) extends Rexp
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
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|
190 |
case class STAR (r: Rexp) extends Rexp
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
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|
191 |
\end{lstlisting}
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Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
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|
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\end{quote}
|
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Christian Urban <christian dot urban at kcl dot ac dot uk>
parents:
diff
changeset
|
193 |
|
229
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
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|
194 |
\noindent In order to be an instance of {\tt Rexp}, each case
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
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|
195 |
object and case class needs to extend {\tt Rexp}. Given the
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
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|
196 |
grammar above, I hope you can see the underlying pattern. If
|
230
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
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|
197 |
you want to play further with such definitions of inductive
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
198 |
datatypes, feel free to define for example binary trees.
|
227
Christian Urban <christian dot urban at kcl dot ac dot uk>
parents:
diff
changeset
|
199 |
|
229
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
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|
200 |
Once you make a definition like the one above, you can
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
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|
201 |
represent, for example, the regular expression for $a + b$ in
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
202 |
Scala as {\tt ALT(CHAR('a'), CHAR('b'))}. Expressions such as
|
230
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
203 |
{\tt 'a'} stand for ASCII characters, though in the output
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
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|
204 |
syntax the quotes are omitted. If you want to assign this
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
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|
205 |
regular expression to a variable, you can use the keyword {\tt
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
206 |
val} and type
|
227
Christian Urban <christian dot urban at kcl dot ac dot uk>
parents:
diff
changeset
|
207 |
|
230
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
208 |
\begin{quote}
|
228
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
209 |
\begin{alltt}
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
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|
210 |
scala> val r = ALT(CHAR('a'), CHAR('b'))
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Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
211 |
r: ALT = ALT(CHAR(a),CHAR(b))
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
212 |
\end{alltt}
|
230
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
213 |
\end{quote}
|
227
Christian Urban <christian dot urban at kcl dot ac dot uk>
parents:
diff
changeset
|
214 |
|
229
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
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|
215 |
\noindent As you can see, in order to make such assignments,
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
216 |
no constructor is required in the class (as in Java). However,
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
217 |
if there is the need for some non-standard initialisation, you
|
230
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
218 |
can of course define such a constructor in Scala too. But we
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
219 |
omit such ``tricks'' here.
|
227
Christian Urban <christian dot urban at kcl dot ac dot uk>
parents:
diff
changeset
|
220 |
|
229
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
221 |
Note that Scala in its response says the variable {\tt r} is
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
222 |
of type {\tt ALT}, not {\tt Rexp}. This might be a bit
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
223 |
unexpected, but can be explained as follows: Scala always
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
224 |
tries to find the most general type that is needed for a
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
225 |
variable or expression, but does not ``over-generalise''. In
|
230
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
226 |
our definition the type {\tt Rexp} is more general than {\tt
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
227 |
ALT}, since it is the abstract class. But in this case there
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
228 |
is no need to give {\tt r} the more general type of {\tt
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
229 |
Rexp}. This is different if you want to form a list of regular
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
230 |
expressions, for example
|
227
Christian Urban <christian dot urban at kcl dot ac dot uk>
parents:
diff
changeset
|
231 |
|
230
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
232 |
\begin{quote}
|
228
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
233 |
\begin{alltt}
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
234 |
scala> val ls = List(ALT(CHAR('a'), CHAR('b')), NULL)
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
235 |
ls: List[Rexp] = List(ALT(CHAR(a),CHAR(b)), NULL)
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
236 |
\end{alltt}
|
230
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
237 |
\end{quote}
|
227
Christian Urban <christian dot urban at kcl dot ac dot uk>
parents:
diff
changeset
|
238 |
|
230
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
239 |
\noindent In this case, Scala needs to assign a common type to
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
240 |
the regular expressions so that it is compatible with the
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
241 |
fact that lists can only contain elements of a single type. In
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
242 |
this case the first common type is {\tt Rexp}.\footnote{If you
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
243 |
type in this example, you will notice that the type contains
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
244 |
some further information, but lets ignore this for the
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
245 |
moment.}
|
229
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
246 |
|
230
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
247 |
For compound types like {\tt List[...]}, the general rule is
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
248 |
that when a type takes another type as argument, then this
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
249 |
argument type is written in angle-brackets. This can also
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
250 |
contain nested types as in {\tt List[Set[Rexp]]}, which is a
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
251 |
list of sets each of which contains regular expressions.
|
227
Christian Urban <christian dot urban at kcl dot ac dot uk>
parents:
diff
changeset
|
252 |
|
228
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
253 |
\subsection*{Functions and Pattern-Matching}
|
227
Christian Urban <christian dot urban at kcl dot ac dot uk>
parents:
diff
changeset
|
254 |
|
229
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
255 |
I mentioned above that Scala is a very elegant programming
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
256 |
language for the code we will write in this module. This
|
230
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
257 |
elegance mainly stems from the fact that in addition to
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
258 |
inductive datatypes, also functions can be implemented very
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
259 |
easily in Scala. To show you this, lets first consider a
|
229
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
260 |
problem from number theory, called the \emph{Collatz-series},
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
261 |
which corresponds to a famous unsolved problem in
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
262 |
mathematics.\footnote{See for example
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
263 |
\url{http://mathworld.wolfram.com/CollatzProblem.html}.}
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
264 |
Mathematician define this series as:
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
265 |
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
266 |
\[
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
267 |
collatz_{n + 1} \dn
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
268 |
\begin{cases}
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
269 |
\frac{1}{2} * collatz_n & \text{if $collatz_n$ is even}\\
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
270 |
3 * collatz_n + 1 & \text{if $collatz_n$ is odd}
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
271 |
\end{cases}
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
272 |
\]
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
273 |
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
274 |
\noindent The famous unsolved question is whether this
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
275 |
series started with any $n > 0$ as $collaz_0$ will always
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
276 |
return to $1$. This is obvious when started with $1$, and also
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
277 |
with $2$, but already needs a bit of head-scratching for the
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
278 |
case of $3$.
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
279 |
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
280 |
If we want to avoid the head-scratching, we could implement
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
281 |
this as the following function in Scala:
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
282 |
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
283 |
\begin{quote}
|
230
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
284 |
\lstinputlisting[language=Scala,numbers=none]{../progs/collatz.scala}
|
229
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
285 |
\end{quote}
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
286 |
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
287 |
\noindent The keyword for function definitions is {\tt def}
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
288 |
followed by the name of the function. After that you have a
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
289 |
list of arguments (enclosed in parentheses and separated by
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
290 |
commas). Each argument in this list needs its type annotated.
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
291 |
In this case we only have one argument, which is of type {\tt
|
230
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
292 |
BigInt}. This type stands in Scala for arbitrary precision
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
293 |
integers (in case you want to try out the function on really
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
294 |
big numbers). After the arguments comes the type of what the
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
295 |
function returns---a Boolean in this case for indicating that
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
296 |
the function has reached {\tt 1}. Finally, after the {\tt =}
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
297 |
comes the \emph{body} of the function implementing what the
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
298 |
function is supposed to do. What the {\tt collatz} function
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
299 |
does should be pretty self-explanatory: the function first
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
300 |
tests whether {\tt n} is equal to $1$ in which case it returns
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
301 |
{\tt true} and so on.
|
229
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
302 |
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
303 |
Notice a quirk in Scala's syntax for {\tt if}s: The condition
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
304 |
needs to be enclosed in parentheses and the then-case comes
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
305 |
right after the condition---there is no {\tt then} keyword in
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
306 |
Scala.
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
307 |
|
230
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
308 |
|
229
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
309 |
The real power of Scala comes, however, from the ability to
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
310 |
define functions by \emph{pattern matching}. In the {\tt
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
311 |
collatz} function above we need to test each case using a
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
312 |
sequence of {\tt if}s. This can be very cumbersome and brittle
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
313 |
if there are many cases. If we wanted to define a function
|
230
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
314 |
over regular expressions in Java, for example, which does not
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
315 |
have pattern-matching, the resulting code would be just
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
316 |
awkward.
|
229
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
317 |
|
230
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
318 |
Mathematicians already use the power of pattern-matching when
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
319 |
they define the function that takes a regular expression and
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
320 |
produces another regular expression that can recognise the
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
321 |
reversed strings. The resulting recursive function is often
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
322 |
defined as follows:
|
229
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
323 |
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
324 |
\begin{center}
|
230
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
325 |
\begin{tabular}{r@{\hspace{2mm}}c@{\hspace{2mm}}l}
|
229
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
326 |
$rev(\varnothing)$ & $\dn$ & $\varnothing$\\
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
327 |
$rev(\epsilon)$ & $\dn$ & $\epsilon$\\
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
328 |
$rev(c)$ & $\dn$ & $c$\\
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
329 |
$rev(r_1 + r_2)$ & $\dn$ & $rev(r_1) + rev(r_2)$\\
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
330 |
$rev(r_1 \cdot r_2)$ & $\dn$ & $rev(r_2) \cdot rev(r_1)$\\
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
331 |
$rev(r^*)$ & $\dn$ & $rev(r)^*$\\
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
332 |
\end{tabular}
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
333 |
\end{center}
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
334 |
|
230
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
335 |
\noindent This function is defined by recursion analysing each
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
336 |
pattern of what the regular expression could look like. The
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
337 |
corresponding Scala code looks very similar to this
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
338 |
definition, thanks to pattern-matching.
|
227
Christian Urban <christian dot urban at kcl dot ac dot uk>
parents:
diff
changeset
|
339 |
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
parents:
diff
changeset
|
340 |
|
229
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
341 |
\begin{quote}
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
342 |
\lstinputlisting[language=Scala]{../progs/rev.scala}
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
343 |
\end{quote}
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
344 |
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
345 |
\noindent The keyword for starting a pattern-match is {\tt
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
346 |
match} followed by a list of {\tt case}s. Before the match
|
230
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
347 |
keyword can be another pattern, but often as in the case
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
348 |
above, it is just a variable you want to pattern-match
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
349 |
(the {\tt r} after {\tt =} in Line 1).
|
229
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
350 |
|
230
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
351 |
Each case in this definition follows the structure of how we
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
352 |
defined regular expressions as inductive datatype. For example
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
353 |
the case in Line 3 you can read as: if the regular expression
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
354 |
{\tt r} is of the form {\tt EMPTY} then do whatever follows
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
355 |
the {\tt =>} (in this case just return {\tt EMPTY}). Line 5
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
356 |
reads as: if the regular expression {\tt r} is of the form
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
357 |
{\tt ALT(r1, r2)}, where the left-branch of the alternative is
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
358 |
matched by the variable {\tt r1} and the right-branch by {\tt
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
359 |
r2} then do ``something''. The ``something'' can now use the
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
360 |
variables {\tt r1} and {\tt r2} from the match.
|
229
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
361 |
|
230
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
362 |
If you want to play with this function, call it for example
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
363 |
with the regular expression $ab + ac$. This regular expression
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
364 |
can recognise the strings $ab$ and $ac$. The function {\tt
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
365 |
rev} produces $ba + ca$, which can recognise the reversed
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
366 |
strings $ba$ and $ca$.
|
229
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
367 |
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
368 |
In Scala each pattern-match can also be guarded as in
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
369 |
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
370 |
\begin{quote}
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
371 |
\begin{lstlisting}[language=Scala, numbers=none]
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
372 |
case Pattern if Condition => Do_Something
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
373 |
\end{lstlisting}
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
374 |
\end{quote}
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
375 |
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
376 |
\noindent This allows us, for example, to re-write the {\tt
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
377 |
collatz}-function from above as follows:
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
378 |
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
379 |
\begin{quote}
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
380 |
\lstinputlisting[language=Scala]{../progs/collatz2.scala}
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
381 |
\end{quote}
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
382 |
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
383 |
\noindent Although in this case the pattern-match does not
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
384 |
improve the code in any way. The underscore in the last case
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
385 |
indicates that we do not care what the pattern looks like.
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
386 |
Thus Line 4 acts like a default case whenever the cases above
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
387 |
did not match. Cases are always tried out from top to bottom.
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
388 |
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
389 |
\subsection*{Loops}
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
390 |
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
391 |
Coming from Java or C, you might be surprised that Scala does
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
392 |
not really have loops. It has instead, what is in functional
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
393 |
programming called \emph{maps}. To illustrate how they work,
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
394 |
lets assume you have a list of numbers from 1 to 10 and want to
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
395 |
build the list of squares. The list of numbers from 1 to 10
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
396 |
can be constructed in Scala as follows:
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
397 |
|
230
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
398 |
\begin{quote}
|
229
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
399 |
\begin{alltt}
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
400 |
scala> (1 to 10).toList
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
401 |
res1: List[Int] = List(1, 2, 3, 4, 5, 6, 7, 8, 9, 10)
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
402 |
\end{alltt}
|
230
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
403 |
\end{quote}
|
229
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
404 |
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
405 |
\noindent Generating from this list the list of squares in a
|
230
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
406 |
non-functional programming language (e.g.~Java), you would
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
407 |
assume the list is given as a kind of array. You would then
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
408 |
iterate, or loop, an index over this array and replace each
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
409 |
entry in the array by the square. Right? In Scala, and in
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
410 |
other functional programming languages, you use maps to
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
411 |
achieve the same.
|
229
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
412 |
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
413 |
Maps essentially take a function that describes how each
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
414 |
element is transformed (for example squaring) and a list over
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
415 |
which this function should work. There are two forms to
|
230
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
416 |
express such maps in Scala. The first way is in a {\tt
|
229
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
417 |
for}-construction. Squaring the numbers from 1 to 10 would
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
418 |
look in this form as follows:
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
419 |
|
230
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
420 |
\begin{quote}
|
229
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
421 |
\begin{alltt}
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
422 |
scala> for (n <- (1 to 10).toList) yield n * n
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
423 |
res2: List[Int] = List(1, 4, 9, 16, 25, 36, 49, 64, 81, 100)
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
424 |
\end{alltt}
|
230
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
425 |
\end{quote}
|
229
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
426 |
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
427 |
\noindent The keywords are {\tt for} and {\tt yield}. This
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
428 |
{\tt for}-construction roughly says that from the list of
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
429 |
numbers we draw {\tt n}s and compute the result of {\tt n *
|
230
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
430 |
n}. As you can see, we specified the list where each {\tt n}
|
229
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
431 |
comes from, namely {\tt (1 to 10).toList}, and how each
|
230
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
432 |
element needs to be transformed. This can also be expressed in
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
433 |
a second way in Scala by using directly {\tt map} as follows:
|
229
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
434 |
|
230
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
435 |
\begin{quote}
|
229
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
436 |
\begin{alltt}
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
437 |
scala> (1 to 10).toList.map(n => n * n)
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
438 |
res3 = List(1, 4, 9, 16, 25, 36, 49, 64, 81, 100)
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
439 |
\end{alltt}
|
230
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
440 |
\end{quote}
|
229
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
441 |
|
230
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
442 |
\noindent In this way, the expression {\tt n => n * n} stands
|
229
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
443 |
for the function that calculates the square (this is how the
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
444 |
{\tt n}s are transformed). This expression for functions might
|
230
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
445 |
remind you of your lessons about the lambda-calculus where
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
446 |
this would have been written as $\lambda n.\,n * n$. It might
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
447 |
not be obvious, but {\tt for}-constructions are just syntactic
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
448 |
sugar: when compiling, Scala translates {\tt
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
449 |
for}-constructions into equivalent maps.
|
229
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
450 |
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
451 |
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
452 |
The very charming feature of Scala is that such maps or {\tt
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
453 |
for}-constructions can be written for any kind of data
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
454 |
collection, such as lists, sets, vectors and so on. For
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
455 |
example if we instead compute the reminders modulo $3$ of this
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
456 |
list, we can write
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
457 |
|
230
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
458 |
\begin{quote}
|
229
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
459 |
\begin{alltt}
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
460 |
scala> (1 to 10).toList.map(n => n \% 3)
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
461 |
res4 = List(1, 2, 0, 1, 2, 0, 1, 2, 0, 1)
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
462 |
\end{alltt}
|
230
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
463 |
\end{quote}
|
229
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
464 |
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
465 |
\noindent If we, however, transform the numbers 1 to 10 not
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
466 |
into a list, but into a set, and then compute the reminders
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
467 |
modulo $3$ we obtain
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
468 |
|
230
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
469 |
\begin{quote}
|
229
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
470 |
\begin{alltt}
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
471 |
scala> (1 to 10).toSet[Int].map(n => n \% 3)
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
472 |
res5 = Set(2, 1, 0)
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
473 |
\end{alltt}
|
230
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
474 |
\end{quote}
|
229
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
475 |
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
476 |
\noindent This is the correct result for sets, as there are
|
230
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
477 |
only three equivalence classes of integers modulo 3. Note that
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
478 |
in this example we need to ``help'' Scala to transform the
|
229
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
479 |
numbers into a set of integers by explicitly annotating the
|
230
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
480 |
type {\tt Int}. Since maps and {\tt for}-constructions are
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
481 |
just syntactic variants of each other, the latter can also be
|
229
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
482 |
written as
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
483 |
|
230
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
484 |
\begin{quote}
|
229
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
485 |
\begin{alltt}
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
486 |
scala> for (n <- (1 to 10).toSet[Int]) yield n \% 3
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
487 |
res5 = Set(2, 1, 0)
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
488 |
\end{alltt}
|
230
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
489 |
\end{quote}
|
229
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
490 |
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
491 |
While hopefully this all looks reasonable, there is one
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
492 |
complication: In the examples above we always wanted to
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
493 |
transform one list into another list (e.g.~list of squares),
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
494 |
or one set into another set (set of numbers into set of
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
495 |
reminders modulo 3). What happens if we just want to print out
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
496 |
a list of integers? Then actually the {\tt for}-construction
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
497 |
needs to be modified. The reason is that {\tt print}, you
|
230
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
498 |
guessed it, does not produce any result, but only produces
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
499 |
what is in the functional-programming-lingo called a
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
500 |
side-effect. Printing out the list of numbers from 1 to 5
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
501 |
would look as follows
|
229
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
502 |
|
230
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
503 |
\begin{quote}
|
229
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
504 |
\begin{alltt}
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
505 |
scala> for (n <- (1 to 5).toList) println(n)
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
506 |
1
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
507 |
2
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
508 |
3
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
509 |
4
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
510 |
5
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
511 |
\end{alltt}
|
230
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
512 |
\end{quote}
|
229
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
513 |
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
514 |
\noindent
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
515 |
where you need to omit the keyword {\tt yield}. You can
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
516 |
also do more elaborate calculations such as
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
517 |
|
230
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
518 |
\begin{quote}
|
229
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
519 |
\begin{alltt}
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
520 |
scala> for (n <- (1 to 5).toList) \{
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
521 |
val square_n = n * n
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
522 |
println(s"$n * $n = $square_n")
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
523 |
\}
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
524 |
1 * 1 = 1
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
525 |
2 * 2 = 4
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
526 |
3 * 3 = 9
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
527 |
4 * 4 = 16
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
528 |
5 * 5 = 25
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
529 |
\end{alltt}
|
230
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
530 |
\end{quote}
|
229
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
531 |
|
230
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
532 |
\noindent In this code I use a variable assignment and a
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
533 |
\emph{string interpolation}, written {\tt s"..."}, in order to
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
534 |
print out an equation. The string interpolation allows me to
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
535 |
refer to the integer values {\tt n} and {\tt square\_n} inside
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
536 |
a string. This is very convenient for printing out ``things''.
|
229
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
537 |
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
538 |
The corresponding map construction for functions with
|
230
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
539 |
side-effects is in Scala called {\tt foreach}. So you
|
229
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
540 |
could also write
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
541 |
|
230
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
542 |
\begin{quote}
|
229
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
543 |
\begin{alltt}
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
544 |
scala> (1 to 5).toList.foreach(n => println(n))
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
545 |
1
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
546 |
2
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
547 |
3
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
548 |
4
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
549 |
5
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
550 |
\end{alltt}
|
230
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
551 |
\end{quote}
|
229
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
552 |
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
553 |
\noindent or even just
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
554 |
|
230
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
555 |
\begin{quote}
|
229
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
556 |
\begin{alltt}
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
557 |
scala> (1 to 5).toList.foreach(println)
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
558 |
1
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
559 |
2
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
560 |
3
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
561 |
4
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
562 |
5
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
563 |
\end{alltt}
|
230
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
564 |
\end{quote}
|
229
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
565 |
|
230
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
566 |
\noindent Again I hope this reminds you a bit of your
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
567 |
lambda-calculus lessons, where an explanation is given why
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
568 |
both forms produce the same result.
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
569 |
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
570 |
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
571 |
If you want to find out more about maps and functions with
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
572 |
side-effects, you can ponder about the response Scala gives if
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
573 |
you replace {\tt foreach} by {\tt map} in the expression
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
574 |
above. Scala will still allow {\tt map} with side-effect
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
575 |
functions, but then reacts with a slightly interesting result.
|
227
Christian Urban <christian dot urban at kcl dot ac dot uk>
parents:
diff
changeset
|
576 |
|
228
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
577 |
\subsection*{Types}
|
227
Christian Urban <christian dot urban at kcl dot ac dot uk>
parents:
diff
changeset
|
578 |
|
229
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
579 |
In most functional programming languages types play an
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
580 |
important role. Scala is such a language. You have already
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
581 |
seen built-in types, like {\tt Int}, {\tt Boolean}, {\tt
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
582 |
String} and {\tt BigInt}, but also user-defined ones, like {\tt Rexp}.
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
583 |
Unfortunately, types can be a thorny subject, especially in
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
584 |
Scala. For example, why do we need to give the type to {\tt
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
585 |
toSet[Int]} but not to {\tt toList}? The reason is the power
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
586 |
of Scala, which sometimes means it cannot infer all necessary
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
587 |
typing information. At the beginning while getting familiar
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
588 |
with Scala, I recommend a ``play-it-by-ear-approach'' to
|
230
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
589 |
types. Fully understanding type-systems, especially complicated
|
229
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
590 |
ones like in Scala, can take a module on their
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
591 |
own.\footnote{Still, such a study can be a rewarding training:
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
592 |
If you are in the business of designing new programming
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
593 |
languages, you will not be able to turn a blind eye to types.
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
594 |
They essentially help programmers to avoid common programming
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
595 |
errors and help with maintaining code.}
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
596 |
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
597 |
In Scala, types are needed whenever you define an inductive
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
598 |
datatype and also whenever you define functions (their
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
599 |
arguments and their results need a type). Base types are types
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
600 |
that do not take any (type)arguments, for example {\tt Int}
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
601 |
and {\tt String}. Compound types take one or more arguments,
|
230
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
602 |
which as seen earlier need to be given in angle-brackets, for
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
603 |
example {\tt List[Int]} or {\tt Set[List[String]]} or {\tt
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
604 |
Map[Int, Int]}.
|
229
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
605 |
|
230
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
606 |
There are a few special type-constructors that fall outside
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
607 |
this pattern. One is for tuples, where the type is written
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
608 |
with parentheses. For example {\tt (Int, Int, String)} for a
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
609 |
triple consisting of two integers and a string. Tuples are
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
610 |
helpful if you want to define functions with multiple
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
611 |
results, say the function returning the quotient and reminder
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
612 |
of two numbers. For this you might define:
|
229
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
613 |
|
230
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
614 |
\begin{quote}
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
615 |
\begin{lstlisting}[language=Scala, numbers=none]
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
616 |
def quo_rem(m: Int, n: Int) : (Int, Int) = (m / n, m \% n)
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
617 |
\end{lstlisting}
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
618 |
\end{quote}
|
229
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
619 |
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
620 |
\noindent
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
621 |
Since this function returns a pair of integers, its type
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
622 |
needs to be {\tt (Int, Int)}.
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
623 |
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
624 |
Another special type-constructor is for functions, written
|
230
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
625 |
as the arrow {\tt =>}. For example, the type {\tt Int =>
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
626 |
String} is for a function that takes an integer as argument
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
627 |
and produces a string. A function of this type is for instance
|
229
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
628 |
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
629 |
\begin{quote}
|
230
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
630 |
\begin{lstlisting}[language=Scala,numbers=none]
|
229
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
631 |
def mk_string(n: Int) : String = n match {
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
632 |
case 0 => "zero"
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
633 |
case 1 => "one"
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
634 |
case 2 => "two"
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
635 |
case _ => "many"
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
636 |
}
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
637 |
\end{lstlisting}
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
638 |
\end{quote}
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
639 |
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
640 |
\noindent Unlike other functional programming languages, there
|
230
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
641 |
is in Scala no easy way to find out the types of existing
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
642 |
functions, except by looking into the documentation
|
229
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
643 |
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
644 |
\begin{quote}
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
645 |
\url{http://www.scala-lang.org/api/current/}
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
646 |
\end{quote}
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
647 |
|
230
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
648 |
The function arrow can also be iterated, as in {\tt
|
229
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
649 |
Int => String => Boolean}. This is the type for a function
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
650 |
taking an integer as first argument and a string as second,
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
651 |
and the result of the function is a boolean. Though silly, a
|
230
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
652 |
function of this type would be
|
229
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
653 |
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
654 |
\begin{quote}
|
230
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
655 |
\begin{lstlisting}[language=Scala,numbers=none]
|
229
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
656 |
def chk_string(n: Int, s: String) : Boolean =
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
657 |
mk_string(n) == s
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
658 |
\end{lstlisting}
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
659 |
\end{quote}
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
660 |
|
230
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
661 |
\noindent which checks whether the integer {\tt n} corresponds
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
662 |
to the name {\tt s} given by the function {\tt mk\_string}.
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
663 |
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
664 |
Coming back to the type {\tt Int => String => Boolean}. The
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
665 |
rule about such function types is that the right-most type
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
666 |
specifies what the function returns (a boolean in this case).
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
667 |
The types before that specify how many arguments the function
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
668 |
expects and what is their type (in this case two arguments,
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
669 |
one of type {\tt Int} and another of type {\tt String}). Given
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
670 |
this rule, what kind of function has type \mbox{\tt (Int =>
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
671 |
String) => Boolean}? Well, it returns a boolean. More
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
672 |
interestingly, though, it only takes a single argument
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
673 |
(because of the parentheses). The single argument happens to
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
674 |
be another function (taking an integer as input and returning
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
675 |
a string).
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
676 |
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
677 |
Now you might ask, what is the point of having function as
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
678 |
arguments to other functions? In Java there is no need of this
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
679 |
kind of feature. But in all functional programming languages,
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
680 |
including Scala, it is really essential. Above you already
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
681 |
seen {\tt map} and {\tt foreach} which need this. Consider
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
682 |
the functions {\tt print} and {\tt println}, which both
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
683 |
print out strings, but the latter adds a line break. You can
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
684 |
call {\tt foreach} with either of them and thus changing how,
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
685 |
for example, five numbers are printed.
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
686 |
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
687 |
\begin{quote}
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
688 |
\begin{alltt}
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
689 |
scala> (1 to 5).toList.foreach(print)
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
690 |
12345
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
691 |
scala> (1 to 5).toList.foreach(println)
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
692 |
1
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
693 |
2
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
694 |
3
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
695 |
4
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
696 |
5
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
697 |
\end{alltt}
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
698 |
\end{quote}
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
699 |
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
700 |
\noindent This is actually one of the main design principles
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
701 |
in functional programming. You have generic functions like
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
702 |
{\tt map} and {\tt foreach} that can traverse data containers,
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
703 |
like lists or sets. They then take a function to specify what
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
704 |
should be done with each element during the traversal. This
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
705 |
requires that the generic traversal functions can cope with
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
706 |
any kind of function (not just functions that, for example,
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
707 |
take as input an integer and produce a string like above).
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
708 |
This means we cannot fix the type of the generic traversal
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
709 |
functions, but have to keep them
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
710 |
\emph{polymorphic}.\footnote{Another interestic topic about
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
711 |
types, but we omit it here for the sake of brevity.}
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
712 |
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
713 |
There is one more type constructor that is rather special. It
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
714 |
is called {\tt Unit}. Recall that {\tt Boolean} has two
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
715 |
values, namely {\tt true} and {\tt false}. This can be used,
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
716 |
for example, to test something and decide whether the test
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
717 |
succeeds or not. In contrast the type {\tt Unit} has only a
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
718 |
single value, written {\tt ()}. This seems like a completely
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
719 |
useless type and return value for a function, but is actually
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
720 |
quite useful. It indicates when the function does not return
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
721 |
any result. The purpose of these functions is to cause
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
722 |
something being written on the screen or written into a file,
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
723 |
for example. This is what is called they cause some effect on
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
724 |
the side, namely a new content displayed on the screen or some
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
725 |
new data in a file. Scala uses the {\tt Unit} type to indicate
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
726 |
that a function does not have a result, but potentially causes
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
727 |
some side-effect. Typical examples are the printing functions,
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
728 |
like {\tt print}.
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
729 |
|
229
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
730 |
|
228
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
731 |
\subsection*{Cool Stuff}
|
227
Christian Urban <christian dot urban at kcl dot ac dot uk>
parents:
diff
changeset
|
732 |
|
230
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
733 |
The first wow-moment I had with Scala when I came across the
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
734 |
following code-snippet for reading a web-page.
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
735 |
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
736 |
\begin{quote}
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
737 |
\begin{lstlisting}[language=Scala, numbers=none]
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
738 |
import io.Source
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
739 |
val url = """http://www.inf.kcl.ac.uk/staff/urbanc/"""
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
740 |
Source.fromURL(url).take(10000).mkString
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
741 |
\end{lstlisting}
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
742 |
\end{quote}
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
743 |
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
744 |
\noindent These three lines return a string containing the
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
745 |
HTML-code of my webpage. It actually already does something
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
746 |
more sophisticated, namely only returns the first 10000
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
747 |
characters of a webpage in case a ``webpage'' is too large.
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
748 |
Why is that code-snippet of any interest? Well, try
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
749 |
implementing reading from a webpage in Java. I also like the
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
750 |
possibility of triple-quoting strings, which I have only seen
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
751 |
in Scala so far. The idea behind this is that in such a string
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
752 |
all characters are interpreted literally---there are no
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
753 |
escaped characters, like \verb|\n| for newlines.
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
754 |
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
755 |
My second wow-moment I had with a feature of Scala that other
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
756 |
functional programming languages do not have. This feature is
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
757 |
about implicit type conversions. If you have regular
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
758 |
expressions and want to use them for language processing you
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
759 |
often want to recognise keywords in a language, for example
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
760 |
{\tt for}, {\tt if}, {\tt yield} and so on. But the basic
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
761 |
regular expression, {\tt CHAR}, can only recognise a single
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
762 |
character. In order to recognise a whole string, like {\tt
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
763 |
for}, you have to put many of those together using {\tt SEQ}:
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
764 |
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
765 |
\begin{quote}
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
766 |
\begin{alltt}
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
767 |
SEQ(CHAR('f'), SEQ(CHAR('o'), CHAR('r')))
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
768 |
\end{alltt}
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
769 |
\end{quote}
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
770 |
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
771 |
\noindent This gets quickly unreadable when the strings and
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
772 |
regular expressions get more complicated. In other functional
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
773 |
programming language, you can explicitly write a conversion
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
774 |
function that takes a string, say {\tt for}, and generates the
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
775 |
regular expression above. But then your code is littered with
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
776 |
such conversion function.
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
777 |
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
778 |
In Scala you can do better by ``hiding'' the conversion
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
779 |
functions. The keyword for doing this is {\tt implicit}.
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
780 |
Consider the code
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
781 |
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
782 |
\begin{quote}
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
783 |
\begin{lstlisting}[language=Scala]
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
784 |
import scala.language.implicitConversions
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
785 |
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
786 |
def charlist2rexp(s: List[Char]) : Rexp = s match {
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
787 |
case Nil => EMPTY
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
788 |
case c::Nil => CHAR(c)
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
789 |
case c::s => SEQ(CHAR(c), charlist2rexp(s))
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
790 |
}
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
791 |
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
792 |
implicit def string2rexp(s: String) : Rexp =
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
793 |
charlist2rexp(s.toList)
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
794 |
\end{lstlisting}
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
795 |
\end{quote}
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
796 |
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
797 |
\noindent where the first seven lines implement a function
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
798 |
that given a list of characters generates the corresponding
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
799 |
regular expression. In Lines 9 and 10, this function is used
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
800 |
for transforming a string into a regular expression. Since the
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
801 |
{\tt string2rexp}-function is declared as {\tt implicit} the
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
802 |
effect will be that whenever Scala expects a regular
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
803 |
expression, but I only give it a string, it will automatically
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
804 |
insert a call to the {\tt string2rexp}-function. I can now
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
805 |
write for example
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
806 |
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
807 |
\begin{quote}
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
808 |
\begin{alltt}
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
809 |
scala> ALT("ab", "ac")
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
810 |
res9: ALT = ALT(SEQ(CHAR(a),CHAR(b)),SEQ(CHAR(a),CHAR(c)))
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
811 |
\end{alltt}
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
812 |
\end{quote}
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
813 |
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
814 |
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
815 |
Using implicit definitions, Scala allows me to introduce
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
816 |
some further syntactic sugar for regular expressions:
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
817 |
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
818 |
\begin{quote}
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
819 |
\begin{lstlisting}[language=Scala]
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
820 |
implicit def RexpOps(r: Rexp) = new {
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
821 |
def | (s: Rexp) = ALT(r, s)
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
822 |
def ~ (s: Rexp) = SEQ(r, s)
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
823 |
def % = STAR(r)
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
824 |
}
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
825 |
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
826 |
implicit def stringOps(s: String) = new {
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
827 |
def | (r: Rexp) = ALT(s, r)
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
828 |
def | (r: String) = ALT(s, r)
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
829 |
def ~ (r: Rexp) = SEQ(s, r)
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
830 |
def ~ (r: String) = SEQ(s, r)
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
831 |
def % = STAR(s)
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
832 |
}
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
833 |
\end{lstlisting}
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
834 |
\end{quote}
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
835 |
|
232
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
836 |
\noindent This might seem a bit overly complicated, but its effect is
|
230
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
837 |
that I can now write regular expressions such as $ab + ac$
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
838 |
even simpler as
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
839 |
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
840 |
\begin{quote}
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
841 |
\begin{alltt}
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
842 |
scala> "ab" | "ac"
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
843 |
res10: ALT = ALT(SEQ(CHAR(a),CHAR(b)),SEQ(CHAR(a),CHAR(c)))
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
844 |
\end{alltt}
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
845 |
\end{quote}
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
846 |
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
847 |
\noindent I leave you to figure out what the other
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
848 |
syntactic sugar in the code above stands for.
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
849 |
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
850 |
One more useful feature of Scala is the ability to define
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
851 |
functions with variable argument lists. This is a feature that
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
852 |
is already present in old languages, like C, but seems to have
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
853 |
been forgotten in the meantime---Java does not have it. In the
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
854 |
context of regular expressions this feature comes in handy:
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
855 |
Say you are fed up with writing many alternatives as
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
856 |
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
857 |
\begin{quote}
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
858 |
\begin{alltt}
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
859 |
ALT(..., ALT(..., ALT(..., ...)))
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
860 |
\end{alltt}
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
861 |
\end{quote}
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
862 |
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
863 |
\noindent To make it difficult, you do not know how deep such
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
864 |
alternatives are nested. So you need something flexible that
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
865 |
can take as many alternatives as needed. In Scala one can
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
866 |
achieve this by adding a {\tt *} to the type of an argument.
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
867 |
Consider the code
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
868 |
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
869 |
\begin{quote}
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
870 |
\begin{lstlisting}[language=Scala]
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
871 |
def Alts(rs: List[Rexp]) : Rexp = rs match {
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
872 |
case Nil => NULL
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
873 |
case r::Nil => r
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
874 |
case r::rs => ALT(r, Alts(rs))
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
875 |
}
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
876 |
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
877 |
def ALTS(rs: Rexp*) = Alts(rs.toList)
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
878 |
\end{lstlisting}
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
879 |
\end{quote}
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
880 |
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
881 |
\noindent The function in Lines 1 to 5 takes a list of regular
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
882 |
expressions and converts it into an appropriate alternative
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
883 |
regular expression. In Line 7 there is a wrapper for this
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
884 |
function which uses the feature of varying argument lists. The
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
885 |
effect of this code is that I can write the regular
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
886 |
expression for keywords as
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
887 |
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
888 |
\begin{quote}
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
889 |
\begin{alltt}
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
890 |
ALTS("for", "def", "yield", "implicit", "if", "match", "case")
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
891 |
\end{alltt}
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
892 |
\end{quote}
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
893 |
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
894 |
\noindent Again I leave you to it how much this simplifies the
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
895 |
regular expression in comparison if I had to write this by
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
896 |
hand using only the ``plain'' regular expressions from the
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
897 |
inductive datatype.
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
898 |
|
229
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
899 |
\subsection*{More Info}
|
227
Christian Urban <christian dot urban at kcl dot ac dot uk>
parents:
diff
changeset
|
900 |
|
232
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
901 |
There is much more to Scala than I can possibly describe in
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
902 |
this document. Fortunately there are a number of free books
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
903 |
about Scala and of course lots of help online. For example
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
904 |
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
905 |
\begin{itemize}
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
906 |
\item \url{http://www.scala-lang.org/docu/files/ScalaByExample.pdf}
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
907 |
\item \url{http://www.scala-lang.org/docu/files/ScalaTutorial.pdf}
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
908 |
\end{itemize}
|
230
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
909 |
|
232
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
910 |
While I am quite enthusiastic about Scala, I am also happy to
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
911 |
admit that it has more than its fair share of faults. The
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
912 |
problem seen earlier of having to give an explicit type to
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
913 |
{\tt toSet}, but not {\tt toList} is one of them. There are
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
914 |
also many ``deep'' ideas about types in Scala, which even to
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
915 |
me as seasoned functional programmer are puzzling. Whilst
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
916 |
implicits are great, they can also be a source of great
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
917 |
headaches, for example consider the code:
|
231
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
918 |
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
919 |
\begin{quote}
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
920 |
\begin{alltt}
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
921 |
scala> List (1, 2, 3) contains "your mom"
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
922 |
res1: Boolean = false
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
923 |
\end{alltt}
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
924 |
\end{quote}
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
925 |
|
232
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
926 |
\noindent Rather than returning {\tt false}, this code should
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
927 |
throw a typing-error. There are also many limitations Scala
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
928 |
inherited from the JVM that can be really annoying. For
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
929 |
example a fixed stack size.
|
231
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
930 |
|
232
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
931 |
Even if Scala has been a success in several high-profile
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
932 |
companies, there is also a company (Yammer) that first used
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
933 |
Scala in their production code, but then moved away from it.
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
934 |
Allegedly they did not like the steep learning curve of Scala
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
935 |
and also that new versions of Scala often introduced
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
936 |
incompatibilities in old code.
|
231
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
937 |
|
232
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
938 |
So all in all, Scala might not be a great teaching language,
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
939 |
but I hope this is mitigated by the fact that I never require
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
940 |
you to write any Scala code. You only need to be able to read
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
941 |
it. In the coursework you can use any programming language you
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
942 |
like. If you want to use Scala for this, then be my guest; if
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
943 |
you do not want, stick with the language you are most familiar
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
944 |
with.
|
230
Christian Urban <christian dot urban at kcl dot ac dot uk>
diff
changeset
|
945 |
|
227
Christian Urban <christian dot urban at kcl dot ac dot uk>
parents:
diff
changeset
|
946 |
\end{document}
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
parents:
diff
changeset
|
947 |
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
parents:
diff
changeset
|
948 |
%%% Local Variables:
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
parents:
diff
changeset
|
949 |
%%% mode: latex
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
parents:
diff
changeset
|
950 |
%%% TeX-master: t
|
Christian Urban <christian dot urban at kcl dot ac dot uk>
parents:
diff
changeset
|
951 |
%%% End:
|