--- a/handouts/scala-ho.tex Thu Aug 28 01:06:09 2014 +0100
+++ b/handouts/scala-ho.tex Sat Aug 30 00:41:52 2014 +0100
@@ -1,17 +1,20 @@
\documentclass{article}
-\usepackage{hyperref}
+\usepackage{xcolor}
+\usepackage{fontspec}
+\usepackage[sc]{mathpazo}
+\usepackage{fontspec}
+\setmainfont[Ligatures=TeX]{Palatino Linotype}
\usepackage{amssymb}
-\usepackage{alltt}
+\usepackage{amsmath}
\usepackage{menukeys}
-\usepackage{amsmath}
\usepackage{../langs}
-\usepackage{mathpazo}
\usepackage{marvosym}
-%%%\usepackage[scaled=.95]{helvet}
+\definecolor{darkblue}{rgb}{0,0,0.6}
+\usepackage[colorlinks=true,urlcolor=darkblue,linkcolor=darkblue]{hyperref}
+
\newcommand{\dn}{\stackrel{\mbox{\scriptsize def}}{=}}%
\definecolor{codegray}{gray}{0.9}
-\newcommand{\code}[1]{\colorbox{codegray}{\texttt{#1}}}
\begin{document}
@@ -26,10 +29,10 @@
Windows.\footnote{There are also experimental backends for
Android and JavaScript.} Unlike Java, however, Scala often
allows programmers to write very concise and elegant code.
-Some therefore say Scala is the much better Java. Some
-companies (The Guardian, Twitter, Coursera, LinkedIn to name a
-few) either use Scala excusively in production code, or some
-part of it are written in Scala. If you want to try out Scala
+Some therefore say Scala is the much better Java. A number of
+companies, The Guardian, Twitter, Coursera, LinkedIn to name a
+few, either use Scala exclusively in production code, or at
+least to some substantial degree. If you want to try out Scala
yourself, the Scala compiler can be downloaded from
\begin{quote}
@@ -37,92 +40,84 @@
\end{quote}
Why do I use Scala in the AFL module? Actually, you can do
-\emph{any} part of the programming coursework in \emph{any}
-programming language you like. I use Scala for showing you
-code during the lectures because its functional
-programming-style allows me to implement the functions we will
-discuss with very small code-snippets. Since the compiler is
-free, you can download them and run every example I give. But
-if you prefer, you can also easily translate the code-snippets
-into any other functional language, for example Haskell,
-Standard ML, F\#, Ocaml and so on.
+\emph{any} part of the coursework in \emph{any} programming
+language you like. I use Scala for showing you code during the
+lectures because its functional programming-style allows me to
+implement the functions we will discuss with very small
+code-snippets. If I had to do this in Java, for example, I
+would first have to run through heaps of boilerplate code.
+Since the Scala compiler is free, you can download the
+code-snippets and run every example I give. But if you prefer,
+you can also easily translate them into any other functional
+language, for example Haskell, Standard ML, F$^\#$, Ocaml and
+so on.
Developing programs in Scala can be done with the Eclipse IDE
and also with IntelliJ IDE, but for the small programs I will
develop the good old Emacs-editor is adequate for me and I
will run the programs on the command line. One advantage of
Scala over Java is that it includes an interpreter (a REPL, or
-Read-Eval-Print-Loop) with which you can run and test small
-code-snippets without the need of the compiler. This helps a
-lot with interactively developing programs. Once you installed
-Scala correctly, you can start the interpreter by typing on
-the command line:
+\underline{R}ead-\underline{E}val-\underline{P}rint-\underline{L}oop)
+with which you can run and test small code-snippets without
+the need of the compiler. This helps a lot with interactively
+developing programs. Once you installed Scala, you can start
+the interpreter by typing on the command line:
-\begin{quote}
-\begin{alltt}
-$ scala\small
+\begin{lstlisting}[language={},numbers=none,basicstyle=\ttfamily\small]
+$ scala
Welcome to Scala version 2.11.2 (Java HotSpot(TM) 64-Bit Server VM).
Type in expressions to have them evaluated.
-Type :help for more information.\normalsize
+Type :help for more information.
scala>
-\end{alltt}
-\end{quote}
+\end{lstlisting}
\noindent The precise response may vary due to the platform
where you installed Scala. At the Scala prompt you can type
-things like {\tt 2 + 3} \keys{Ret} and the output will be
+things like \code{2 + 3} \keys{Ret} and the output will be
-\begin{quote}
-\begin{alltt}
+\begin{lstlisting}[language=Scala,numbers=none]
scala> 2 + 3
res0: Int = 5
-\end{alltt}
-\end{quote}
+\end{lstlisting}
\noindent indicating that the result of the addition is of
-type {\tt Int} and the actual result is {\tt 5}. Another
-classic example you can try out is
+type \code{Int} and the actual result is 5. Another classic
+example you can try out is
-\begin{quote}
-\begin{alltt}
-scala> print ("hello world")
+\begin{lstlisting}[language=Scala,numbers=none]
+scala> print("hello world")
hello world
-\end{alltt}
-\end{quote}
+\end{lstlisting}
\noindent Note that in this case there is no result. The
-reason is that {\tt print} does not actually produce a result
-(there is no {\tt resXX}), rather it is a function that causes
-the \emph{side-effect} of printing out a string. Once you are
-more familiar with the functional programming-style, you will
-know what the difference is between a function that returns a
-result, like addition, and a function that causes a
-side-effect, like {\tt print}. We shall come back to this
+reason is that \code{print} does not actually produce a result
+(there is no \code{resXX}), rather it is a function that
+causes the \emph{side-effect} of printing out a string. Once
+you are more familiar with the functional programming-style,
+you will know what the difference is between a function that
+returns a result, like addition, and a function that causes a
+side-effect, like \code{print}. We shall come back to this
point later, but if you are curious now, the latter kind of
-functions always have as return type {\tt Unit}.
+functions always have as return type \code{Unit}.
If you want to write a stand-alone app in Scala, you can
-implement an object that is an instance of {\tt App}, say
+implement an object that is an instance of \code{App}, say
-\begin{quote}
\begin{lstlisting}[language=Scala,numbers=none]
object Hello extends App {
println ("hello world")
}
\end{lstlisting}
-\end{quote}
-\noindent save it in a file, say {\tt hellow-world.scala}, and
+\noindent save it in a file, say {\tt hello-world.scala}, and
then run the compiler and runtime environment:
-\begin{quote}
-\begin{alltt}
+\begin{lstlisting}[language={},numbers=none,basicstyle=\ttfamily\small]
$ scalac hello-world.scala
$ scala Hello
hello world
-\end{alltt}
-\end{quote}
+\end{lstlisting}
As mentioned above, Scala targets the JVM and consequently
Scala programs can also be executed by the bog-standard Java
@@ -130,20 +125,18 @@
scala-library.jar}, which on my computer can be done as
follows:
-\begin{quote}
-\begin{alltt}
+\begin{lstlisting}[language={},numbers=none,basicstyle=\ttfamily\small]
$ scalac hello-world.scala
$ java -cp /usr/local/src/scala/lib/scala-library.jar:. Hello
hello world
-\end{alltt}
-\end{quote}
+\end{lstlisting}
\subsection*{Inductive Datatypes}
The elegance and conciseness of Scala programs are often a
result of inductive datatypes that can be easily defined. For
-example in ``every-day mathematics'' we would define regular
+example in ``every-day mathematics'' we define regular
expressions simply by giving the grammar
\begin{center}
@@ -163,23 +156,22 @@
of leave nodes---null, empty and character). If you are
familiar with Java, it might be an instructive exercise to
define this kind of inductive datatypes in Java\footnote{Happy
-programming! \Smiley} and then compare it how
-it can be defined in Scala.
+programming! \Smiley} and then compare it with how it can be
+defined in Scala.
Implementing the regular expressions from above in Scala is
actually very simple: It first requires an \emph{abstract
-class}, say, {\tt Rexp}. This will act as the type for regular
-expressions. Second, it requires a case for each clause in the
-grammar. The cases for $\varnothing$ and $\epsilon$ do not
-have any arguments, while in all the other cases we do have
-arguments. For example the character regular expression needs
-to take as an argument the character it is supposed to
-recognise. In Scala, the cases without arguments are called
-\emph{case objects}, while the ones with arguments are
-\emph{case classes}. The corresponding Scala code is as
+class}, say, \code{Rexp}. This will act as the type for
+regular expressions. Second, it requires a case for each
+clause in the grammar. The cases for $\varnothing$ and
+$\epsilon$ do not have any arguments, while in all the other
+cases we do have arguments. For example the character regular
+expression needs to take as an argument the character it is
+supposed to recognise. In Scala, the cases without arguments
+are called \emph{case objects}, while the ones with arguments
+are \emph{case classes}. The corresponding Scala code is as
follows:
-\begin{quote}
\begin{lstlisting}[language=Scala,numbers=none]
abstract class Rexp
case object NULL extends Rexp
@@ -189,28 +181,25 @@
case class ALT (r1: Rexp, r2: Rexp) extends Rexp
case class STAR (r: Rexp) extends Rexp
\end{lstlisting}
-\end{quote}
-\noindent In order to be an instance of {\tt Rexp}, each case
-object and case class needs to extend {\tt Rexp}. Given the
+\noindent In order to be an instance of \code{Rexp}, each case
+object and case class needs to extend \code{Rexp}. Given the
grammar above, I hope you can see the underlying pattern. If
you want to play further with such definitions of inductive
datatypes, feel free to define for example binary trees.
-Once you make a definition like the one above, you can
-represent, for example, the regular expression for $a + b$ in
-Scala as {\tt ALT(CHAR('a'), CHAR('b'))}. Expressions such as
-{\tt 'a'} stand for ASCII characters, though in the output
+Once you make a definition like the one above in Scala, you
+can represent, for example, the regular expression for $a + b$
+as \code{ALT(CHAR('a'), CHAR('b'))}. Expressions such as
+\code{'a'} stand for ASCII characters, though in the output
syntax the quotes are omitted. If you want to assign this
-regular expression to a variable, you can use the keyword {\tt
-val} and type
+regular expression to a variable, you can use the keyword
+\code{val} and type
-\begin{quote}
-\begin{alltt}
+\begin{lstlisting}[language=Scala,numbers=none]
scala> val r = ALT(CHAR('a'), CHAR('b'))
r: ALT = ALT(CHAR(a),CHAR(b))
-\end{alltt}
-\end{quote}
+\end{lstlisting}
\noindent As you can see, in order to make such assignments,
no constructor is required in the class (as in Java). However,
@@ -218,36 +207,37 @@
can of course define such a constructor in Scala too. But we
omit such ``tricks'' here.
-Note that Scala in its response says the variable {\tt r} is
-of type {\tt ALT}, not {\tt Rexp}. This might be a bit
-unexpected, but can be explained as follows: Scala always
-tries to find the most general type that is needed for a
-variable or expression, but does not ``over-generalise''. In
-our definition the type {\tt Rexp} is more general than {\tt
-ALT}, since it is the abstract class. But in this case there
-is no need to give {\tt r} the more general type of {\tt
-Rexp}. This is different if you want to form a list of regular
-expressions, for example
+Note that Scala in its response says the variable \code{r} is
+of type \lstinline[emph={ALT}]!ALT!, not \code{Rexp}. This
+might be a bit unexpected, but can be explained as follows:
+Scala always tries to find the most general type that is
+needed for a variable or expression, but does not
+``over-generalise''. In our definition the type \code{Rexp} is
+more general than \lstinline[emph={ALT}]!ALT!, since it is the
+abstract class. But in this case there is no need to give
+\code{r} the more general type of \code{Rexp}. This is
+different if you want to form a list of regular expressions,
+for example
-\begin{quote}
-\begin{alltt}
+
+\begin{lstlisting}[language=Scala,numbers=none]
scala> val ls = List(ALT(CHAR('a'), CHAR('b')), NULL)
ls: List[Rexp] = List(ALT(CHAR(a),CHAR(b)), NULL)
-\end{alltt}
-\end{quote}
+\end{lstlisting}
+
\noindent In this case, Scala needs to assign a common type to
the regular expressions so that it is compatible with the
fact that lists can only contain elements of a single type. In
-this case the first common type is {\tt Rexp}.\footnote{If you
+this case the first common type is \code{Rexp}.\footnote{If you
type in this example, you will notice that the type contains
some further information, but lets ignore this for the
moment.}
-For compound types like {\tt List[...]}, the general rule is
+For compound types like \code{List[...]}, the general rule is
that when a type takes another type as argument, then this
argument type is written in angle-brackets. This can also
-contain nested types as in {\tt List[Set[Rexp]]}, which is a
+contain nested types as in \code{List[Set[Rexp]]}, which is a
list of sets each of which contains regular expressions.
\subsection*{Functions and Pattern-Matching}
@@ -280,36 +270,36 @@
If we want to avoid the head-scratching, we could implement
this as the following function in Scala:
-\begin{quote}
+
\lstinputlisting[language=Scala,numbers=none]{../progs/collatz.scala}
-\end{quote}
+
-\noindent The keyword for function definitions is {\tt def}
+\noindent The keyword for function definitions is \code{def}
followed by the name of the function. After that you have a
list of arguments (enclosed in parentheses and separated by
commas). Each argument in this list needs its type annotated.
-In this case we only have one argument, which is of type {\tt
-BigInt}. This type stands in Scala for arbitrary precision
+In this case we only have one argument, which is of type
+\code{BigInt}. This type stands in Scala for arbitrary precision
integers (in case you want to try out the function on really
big numbers). After the arguments comes the type of what the
function returns---a Boolean in this case for indicating that
-the function has reached {\tt 1}. Finally, after the {\tt =}
+the function has reached 1. Finally, after the \code{=}
comes the \emph{body} of the function implementing what the
-function is supposed to do. What the {\tt collatz} function
+function is supposed to do. What the \code{collatz} function
does should be pretty self-explanatory: the function first
-tests whether {\tt n} is equal to $1$ in which case it returns
-{\tt true} and so on.
+tests whether \code{n} is equal to 1 in which case it returns
+\code{true} and so on.
-Notice a quirk in Scala's syntax for {\tt if}s: The condition
+Notice a quirk in Scala's syntax for \code{if}s: The condition
needs to be enclosed in parentheses and the then-case comes
-right after the condition---there is no {\tt then} keyword in
+right after the condition---there is no \code{then} keyword in
Scala.
The real power of Scala comes, however, from the ability to
-define functions by \emph{pattern matching}. In the {\tt
-collatz} function above we need to test each case using a
-sequence of {\tt if}s. This can be very cumbersome and brittle
+define functions by \emph{pattern matching}. In the
+\code{collatz} function above we need to test each case using a
+sequence of \code{if}s. This can be very cumbersome and brittle
if there are many cases. If we wanted to define a function
over regular expressions in Java, for example, which does not
have pattern-matching, the resulting code would be just
@@ -337,56 +327,51 @@
corresponding Scala code looks very similar to this
definition, thanks to pattern-matching.
+\lstinputlisting[language=Scala]{../progs/rev.scala}
-\begin{quote}
-\lstinputlisting[language=Scala]{../progs/rev.scala}
-\end{quote}
-
-\noindent The keyword for starting a pattern-match is {\tt
-match} followed by a list of {\tt case}s. Before the match
+\noindent The keyword for starting a pattern-match is
+\code{match} followed by a list of \code{case}s. Before the match
keyword can be another pattern, but often as in the case
above, it is just a variable you want to pattern-match
-(the {\tt r} after {\tt =} in Line 1).
+(the \code{r} after \code{=} in Line 1).
Each case in this definition follows the structure of how we
defined regular expressions as inductive datatype. For example
the case in Line 3 you can read as: if the regular expression
-{\tt r} is of the form {\tt EMPTY} then do whatever follows
-the {\tt =>} (in this case just return {\tt EMPTY}). Line 5
-reads as: if the regular expression {\tt r} is of the form
-{\tt ALT(r1, r2)}, where the left-branch of the alternative is
-matched by the variable {\tt r1} and the right-branch by {\tt
-r2} then do ``something''. The ``something'' can now use the
-variables {\tt r1} and {\tt r2} from the match.
+\code{r} is of the form \code{EMPTY} then do whatever follows
+the \code{=>} (in this case just return \code{EMPTY}). Line 5
+reads as: if the regular expression \code{r} is of the form
+\code{ALT(r1, r2)}, where the left-branch of the alternative is
+matched by the variable \code{r1} and the right-branch by
+\code{r2} then do ``something''. The ``something'' can now use the
+variables \code{r1} and \code{r2} from the match.
If you want to play with this function, call it for example
with the regular expression $ab + ac$. This regular expression
-can recognise the strings $ab$ and $ac$. The function {\tt
-rev} produces $ba + ca$, which can recognise the reversed
+can recognise the strings $ab$ and $ac$. The function
+\code{rev} produces $ba + ca$, which can recognise the reversed
strings $ba$ and $ca$.
In Scala each pattern-match can also be guarded as in
-\begin{quote}
\begin{lstlisting}[language=Scala, numbers=none]
case Pattern if Condition => Do_Something
\end{lstlisting}
-\end{quote}
-\noindent This allows us, for example, to re-write the {\tt
-collatz}-function from above as follows:
+\noindent This allows us, for example, to re-write the
+\code{collatz}-function from above as follows:
-\begin{quote}
\lstinputlisting[language=Scala]{../progs/collatz2.scala}
-\end{quote}
+
\noindent Although in this case the pattern-match does not
-improve the code in any way. The underscore in the last case
-indicates that we do not care what the pattern looks like.
-Thus Line 4 acts like a default case whenever the cases above
-did not match. Cases are always tried out from top to bottom.
+improve the code in any way. In cases like \code{rev} it
+is really crucial. The underscore in the last case indicates
+that we do not care what the pattern looks like. Thus Line 4
+acts like a default case whenever the cases above did not
+match. Cases are always tried out from top to bottom.
-\subsection*{Loops}
+\subsection*{Loops, or the Absence of}
Coming from Java or C, you might be surprised that Scala does
not really have loops. It has instead, what is in functional
@@ -395,173 +380,184 @@
build the list of squares. The list of numbers from 1 to 10
can be constructed in Scala as follows:
-\begin{quote}
-\begin{alltt}
+\begin{lstlisting}[language=Scala,numbers=none]
scala> (1 to 10).toList
res1: List[Int] = List(1, 2, 3, 4, 5, 6, 7, 8, 9, 10)
-\end{alltt}
-\end{quote}
+\end{lstlisting}
\noindent Generating from this list the list of squares in a
-non-functional programming language (e.g.~Java), you would
-assume the list is given as a kind of array. You would then
-iterate, or loop, an index over this array and replace each
-entry in the array by the square. Right? In Scala, and in
-other functional programming languages, you use maps to
-achieve the same.
+programming language such as Java, you would assume the list
+is given as a kind of array. You would then iterate, or loop,
+an index over this array and replace each entry in the array
+by the square. Right? In Scala, and in other functional
+programming languages, you use maps to achieve the same.
-Maps essentially take a function that describes how each
-element is transformed (for example squaring) and a list over
+A map essentially takes a function that describes how each
+element is transformed (for example squared) and a list over
which this function should work. There are two forms to
-express such maps in Scala. The first way is in a {\tt
-for}-construction. Squaring the numbers from 1 to 10 would
+express such maps in Scala. The first way is called a
+for-comprehension. Squaring the numbers from 1 to 10 would
look in this form as follows:
-\begin{quote}
-\begin{alltt}
+\begin{lstlisting}[language=Scala,numbers=none]
scala> for (n <- (1 to 10).toList) yield n * n
res2: List[Int] = List(1, 4, 9, 16, 25, 36, 49, 64, 81, 100)
-\end{alltt}
-\end{quote}
+\end{lstlisting}
-\noindent The keywords are {\tt for} and {\tt yield}. This
-{\tt for}-construction roughly says that from the list of
-numbers we draw {\tt n}s and compute the result of {\tt n *
-n}. As you can see, we specified the list where each {\tt n}
-comes from, namely {\tt (1 to 10).toList}, and how each
-element needs to be transformed. This can also be expressed in
-a second way in Scala by using directly {\tt map} as follows:
+\noindent The important keywords are \code{for} and
+\code{yield}. This for-comprehension roughly states that from
+the list of numbers we draw \code{n}s and compute the result
+of \code{n * n}. As you can see, we specified the list where
+each \code{n} comes from, namely \code{(1 to 10).toList}, and
+how each element needs to be transformed. This can also be
+expressed in a second way in Scala by using directly
+\code{map}s as follows:
-\begin{quote}
-\begin{alltt}
+\begin{lstlisting}[language=Scala,numbers=none]
scala> (1 to 10).toList.map(n => n * n)
res3 = List(1, 4, 9, 16, 25, 36, 49, 64, 81, 100)
-\end{alltt}
-\end{quote}
-
-\noindent In this way, the expression {\tt n => n * n} stands
-for the function that calculates the square (this is how the
-{\tt n}s are transformed). This expression for functions might
-remind you of your lessons about the lambda-calculus where
-this would have been written as $\lambda n.\,n * n$. It might
-not be obvious, but {\tt for}-constructions are just syntactic
-sugar: when compiling, Scala translates {\tt
-for}-constructions into equivalent maps.
-
+\end{lstlisting}
-The very charming feature of Scala is that such maps or {\tt
-for}-constructions can be written for any kind of data
-collection, such as lists, sets, vectors and so on. For
-example if we instead compute the reminders modulo $3$ of this
-list, we can write
+\noindent In this way, the expression \code{n => n * n} stands
+for the function that calculates the square (this is how the
+\code{n}s are transformed). This expression for functions
+might remind you of your lessons about the lambda-calculus
+where this would have been written as $\lambda n.\,n * n$. It
+might not be obvious, but for-comprehensions are just
+syntactic sugar: when compiling, Scala translates
+for-comprehensions into equivalent maps. This even works
+when for-comprehensions get more complicated (see below).
-\begin{quote}
-\begin{alltt}
-scala> (1 to 10).toList.map(n => n \% 3)
+The very charming feature of Scala is that such maps or
+for-comprehensions can be written for any kind of data
+collection, such as lists, sets, vectors, options and so on.
+For example if we instead compute the reminders modulo 3 of
+this list, we can write
+
+\begin{lstlisting}[language=Scala,numbers=none]
+scala> (1 to 10).toList.map(n => n % 3)
res4 = List(1, 2, 0, 1, 2, 0, 1, 2, 0, 1)
-\end{alltt}
-\end{quote}
+\end{lstlisting}
\noindent If we, however, transform the numbers 1 to 10 not
into a list, but into a set, and then compute the reminders
-modulo $3$ we obtain
+modulo 3 we obtain
-\begin{quote}
-\begin{alltt}
-scala> (1 to 10).toSet[Int].map(n => n \% 3)
+\begin{lstlisting}[language=Scala,numbers=none]
+scala> (1 to 10).toSet[Int].map(n => n % 3)
res5 = Set(2, 1, 0)
-\end{alltt}
-\end{quote}
+\end{lstlisting}
\noindent This is the correct result for sets, as there are
only three equivalence classes of integers modulo 3. Note that
in this example we need to ``help'' Scala to transform the
numbers into a set of integers by explicitly annotating the
-type {\tt Int}. Since maps and {\tt for}-constructions are
+type \code{Int}. Since maps and for-comprehensions are
just syntactic variants of each other, the latter can also be
written as
-\begin{quote}
-\begin{alltt}
-scala> for (n <- (1 to 10).toSet[Int]) yield n \% 3
+\begin{lstlisting}[language=Scala,numbers=none]
+scala> for (n <- (1 to 10).toSet[Int]) yield n % 3
res5 = Set(2, 1, 0)
-\end{alltt}
-\end{quote}
+\end{lstlisting}
+
+For-comprehensions can also be nested and the selection of
+elements can be guarded. For example if we want to pair up
+the numbers 1 to 4 with the letters a to c, we can write
+
+\begin{lstlisting}[language=Scala,numbers=none]
+scala> for (n <- (1 to 4).toList;
+ c <- ('a' to 'c').toList) yield (n, c)
+res6 = List((1,a), (1,b), (1,c), (2,a), (2,b), (2,c),
+ (3,a), (3,b), (3,c), (4,a), (4,b), (4,c))
+\end{lstlisting}
+
+\noindent
+Or if we want to find all pairs of numbers between 1 and 3
+where the sum is an even number, we can write
+
+\begin{lstlisting}[language=Scala,numbers=none]
+scala> for (n <- (1 to 3).toList;
+ m <- (1 to 3).toList;
+ if (n + m) % 2 == 0) yield (n, m)
+res7 = List((1,1), (1,3), (2,2), (3,1), (3,3))
+\end{lstlisting}
+
+\noindent
+The \code{if}-condition filters out all pairs where the
+sum is not even.
While hopefully this all looks reasonable, there is one
complication: In the examples above we always wanted to
transform one list into another list (e.g.~list of squares),
or one set into another set (set of numbers into set of
reminders modulo 3). What happens if we just want to print out
-a list of integers? Then actually the {\tt for}-construction
-needs to be modified. The reason is that {\tt print}, you
+a list of integers? Then actually the for-comprehension
+needs to be modified. The reason is that \code{print}, you
guessed it, does not produce any result, but only produces
what is in the functional-programming-lingo called a
side-effect. Printing out the list of numbers from 1 to 5
would look as follows
-\begin{quote}
-\begin{alltt}
+\begin{lstlisting}[language=Scala,numbers=none]
scala> for (n <- (1 to 5).toList) println(n)
1
2
3
4
5
-\end{alltt}
-\end{quote}
+\end{lstlisting}
\noindent
-where you need to omit the keyword {\tt yield}. You can
+where you need to omit the keyword \code{yield}. You can
also do more elaborate calculations such as
-\begin{quote}
-\begin{alltt}
-scala> for (n <- (1 to 5).toList) \{
+\begin{lstlisting}[language=Scala,numbers=none]
+scala> for (n <- (1 to 5).toList) {
val square_n = n * n
println(s"$n * $n = $square_n")
-\}
+}
1 * 1 = 1
2 * 2 = 4
3 * 3 = 9
4 * 4 = 16
5 * 5 = 25
-\end{alltt}
-\end{quote}
+\end{lstlisting}
-\noindent In this code I use a variable assignment and a
-\emph{string interpolation}, written {\tt s"..."}, in order to
+\noindent In this code I use a variable assignment (\code{val
+square_n = ...} ) and what is called a
+\emph{string interpolation}, written \code{s"..."}, in order to
print out an equation. The string interpolation allows me to
-refer to the integer values {\tt n} and {\tt square\_n} inside
+refer to the integer values \code{n} and \code{square\_n} inside
a string. This is very convenient for printing out ``things''.
The corresponding map construction for functions with
-side-effects is in Scala called {\tt foreach}. So you
+side-effects is in Scala called \code{foreach}. So you
could also write
-\begin{quote}
-\begin{alltt}
+
+\begin{lstlisting}[language=Scala,numbers=none]
scala> (1 to 5).toList.foreach(n => println(n))
1
2
3
4
5
-\end{alltt}
-\end{quote}
+\end{lstlisting}
+
\noindent or even just
-\begin{quote}
-\begin{alltt}
+
+\begin{lstlisting}[language=Scala,numbers=none]
scala> (1 to 5).toList.foreach(println)
1
2
3
4
5
-\end{alltt}
-\end{quote}
+\end{lstlisting}
+
\noindent Again I hope this reminds you a bit of your
lambda-calculus lessons, where an explanation is given why
@@ -570,63 +566,64 @@
If you want to find out more about maps and functions with
side-effects, you can ponder about the response Scala gives if
-you replace {\tt foreach} by {\tt map} in the expression
-above. Scala will still allow {\tt map} with side-effect
+you replace \code{foreach} by \code{map} in the expression
+above. Scala will still allow \code{map} with side-effect
functions, but then reacts with a slightly interesting result.
\subsection*{Types}
In most functional programming languages types play an
important role. Scala is such a language. You have already
-seen built-in types, like {\tt Int}, {\tt Boolean}, {\tt
-String} and {\tt BigInt}, but also user-defined ones, like {\tt Rexp}.
-Unfortunately, types can be a thorny subject, especially in
-Scala. For example, why do we need to give the type to {\tt
-toSet[Int]} but not to {\tt toList}? The reason is the power
-of Scala, which sometimes means it cannot infer all necessary
-typing information. At the beginning while getting familiar
-with Scala, I recommend a ``play-it-by-ear-approach'' to
-types. Fully understanding type-systems, especially complicated
-ones like in Scala, can take a module on their
-own.\footnote{Still, such a study can be a rewarding training:
-If you are in the business of designing new programming
-languages, you will not be able to turn a blind eye to types.
-They essentially help programmers to avoid common programming
-errors and help with maintaining code.}
+seen built-in types, like \code{Int}, \code{Boolean},
+\code{String} and \code{BigInt}, but also user-defined ones,
+like \code{Rexp}. Unfortunately, types can be a thorny
+subject, especially in Scala. For example, why do we need to
+give the type to \code{toSet[Int]} but not to \code{toList}?
+The reason is the power of Scala, which sometimes means it
+cannot infer all necessary typing information. At the
+beginning while getting familiar with Scala, I recommend a
+``play-it-by-ear-approach'' to types. Fully understanding
+type-systems, especially complicated ones like in Scala, can
+take a module on their own.\footnote{Still, such a study can
+be a rewarding training: If you are in the business of
+designing new programming languages, you will not be able to
+turn a blind eye to types. They essentially help programmers
+to avoid common programming errors and help with maintaining
+code.}
In Scala, types are needed whenever you define an inductive
datatype and also whenever you define functions (their
arguments and their results need a type). Base types are types
-that do not take any (type)arguments, for example {\tt Int}
-and {\tt String}. Compound types take one or more arguments,
+that do not take any (type)arguments, for example \code{Int}
+and \code{String}. Compound types take one or more arguments,
which as seen earlier need to be given in angle-brackets, for
-example {\tt List[Int]} or {\tt Set[List[String]]} or {\tt
-Map[Int, Int]}.
+example \code{List[Int]} or \code{Set[List[String]]} or
+\code{Map[Int, Int]}.
There are a few special type-constructors that fall outside
this pattern. One is for tuples, where the type is written
-with parentheses. For example {\tt (Int, Int, String)} for a
+with parentheses. For example \code{(Int, Int, String)} for a
triple consisting of two integers and a string. Tuples are
helpful if you want to define functions with multiple
results, say the function returning the quotient and reminder
of two numbers. For this you might define:
-\begin{quote}
+
\begin{lstlisting}[language=Scala, numbers=none]
def quo_rem(m: Int, n: Int) : (Int, Int) = (m / n, m \% n)
\end{lstlisting}
-\end{quote}
+
\noindent
Since this function returns a pair of integers, its type
-needs to be {\tt (Int, Int)}.
+needs to be \code{(Int, Int)}.
Another special type-constructor is for functions, written
-as the arrow {\tt =>}. For example, the type {\tt Int =>
+as the arrow \code{=>}. For example, the type \code{Int =>
String} is for a function that takes an integer as argument
and produces a string. A function of this type is for instance
-\begin{quote}
+
\begin{lstlisting}[language=Scala,numbers=none]
def mk_string(n: Int) : String = n match {
case 0 => "zero"
@@ -635,7 +632,7 @@
case _ => "many"
}
\end{lstlisting}
-\end{quote}
+
\noindent Unlike other functional programming languages, there
is in Scala no easy way to find out the types of existing
@@ -645,47 +642,47 @@
\url{http://www.scala-lang.org/api/current/}
\end{quote}
-The function arrow can also be iterated, as in {\tt
-Int => String => Boolean}. This is the type for a function
+The function arrow can also be iterated, as in
+\code{Int => String => Boolean}. This is the type for a function
taking an integer as first argument and a string as second,
and the result of the function is a boolean. Though silly, a
function of this type would be
-\begin{quote}
+
\begin{lstlisting}[language=Scala,numbers=none]
def chk_string(n: Int, s: String) : Boolean =
mk_string(n) == s
\end{lstlisting}
-\end{quote}
+
-\noindent which checks whether the integer {\tt n} corresponds
-to the name {\tt s} given by the function {\tt mk\_string}.
+\noindent which checks whether the integer \code{n} corresponds
+to the name \code{s} given by the function \code{mk\_string}.
-Coming back to the type {\tt Int => String => Boolean}. The
+Coming back to the type \code{Int => String => Boolean}. The
rule about such function types is that the right-most type
specifies what the function returns (a boolean in this case).
The types before that specify how many arguments the function
expects and what is their type (in this case two arguments,
-one of type {\tt Int} and another of type {\tt String}). Given
-this rule, what kind of function has type \mbox{\tt (Int =>
-String) => Boolean}? Well, it returns a boolean. More
-interestingly, though, it only takes a single argument
-(because of the parentheses). The single argument happens to
-be another function (taking an integer as input and returning
-a string).
+one of type \code{Int} and another of type \code{String}).
+Given this rule, what kind of function has type
+\mbox{\code{(Int => String) => Boolean}}? Well, it returns a
+boolean. More interestingly, though, it only takes a single
+argument (because of the parentheses). The single argument
+happens to be another function (taking an integer as input and
+returning a string).
Now you might ask, what is the point of having function as
arguments to other functions? In Java there is no need of this
kind of feature. But in all functional programming languages,
including Scala, it is really essential. Above you already
-seen {\tt map} and {\tt foreach} which need this. Consider
-the functions {\tt print} and {\tt println}, which both
+seen \code{map} and \code{foreach} which need this. Consider
+the functions \code{print} and \code{println}, which both
print out strings, but the latter adds a line break. You can
-call {\tt foreach} with either of them and thus changing how,
+call \code{foreach} with either of them and thus changing how,
for example, five numbers are printed.
-\begin{quote}
-\begin{alltt}
+
+\begin{lstlisting}[language=Scala,numbers=none]
scala> (1 to 5).toList.foreach(print)
12345
scala> (1 to 5).toList.foreach(println)
@@ -694,12 +691,12 @@
3
4
5
-\end{alltt}
-\end{quote}
+\end{lstlisting}
+
\noindent This is actually one of the main design principles
in functional programming. You have generic functions like
-{\tt map} and {\tt foreach} that can traverse data containers,
+\code{map} and \code{foreach} that can traverse data containers,
like lists or sets. They then take a function to specify what
should be done with each element during the traversal. This
requires that the generic traversal functions can cope with
@@ -711,21 +708,21 @@
types, but we omit it here for the sake of brevity.}
There is one more type constructor that is rather special. It
-is called {\tt Unit}. Recall that {\tt Boolean} has two
-values, namely {\tt true} and {\tt false}. This can be used,
+is called \code{Unit}. Recall that \code{Boolean} has two
+values, namely \code{true} and \code{false}. This can be used,
for example, to test something and decide whether the test
-succeeds or not. In contrast the type {\tt Unit} has only a
-single value, written {\tt ()}. This seems like a completely
+succeeds or not. In contrast the type \code{Unit} has only a
+single value, written \code{()}. This seems like a completely
useless type and return value for a function, but is actually
quite useful. It indicates when the function does not return
any result. The purpose of these functions is to cause
something being written on the screen or written into a file,
for example. This is what is called they cause some effect on
the side, namely a new content displayed on the screen or some
-new data in a file. Scala uses the {\tt Unit} type to indicate
+new data in a file. Scala uses the \code{Unit} type to indicate
that a function does not have a result, but potentially causes
some side-effect. Typical examples are the printing functions,
-like {\tt print}.
+like \code{print}.
\subsection*{Cool Stuff}
@@ -733,13 +730,13 @@
The first wow-moment I had with Scala when I came across the
following code-snippet for reading a web-page.
-\begin{quote}
+
\begin{lstlisting}[language=Scala, numbers=none]
import io.Source
val url = """http://www.inf.kcl.ac.uk/staff/urbanc/"""
Source.fromURL(url).take(10000).mkString
\end{lstlisting}
-\end{quote}
+
\noindent These three lines return a string containing the
HTML-code of my webpage. It actually already does something
@@ -757,29 +754,29 @@
about implicit type conversions. If you have regular
expressions and want to use them for language processing you
often want to recognise keywords in a language, for example
-{\tt for}, {\tt if}, {\tt yield} and so on. But the basic
-regular expression, {\tt CHAR}, can only recognise a single
-character. In order to recognise a whole string, like {\tt
-for}, you have to put many of those together using {\tt SEQ}:
+\code{for}, \code{if}, \code{yield} and so on. But the basic
+regular expression, \code{CHAR}, can only recognise a single
+character. In order to recognise a whole string, like \code{
+for}, you have to put many of those together using \code{SEQ}:
-\begin{quote}
-\begin{alltt}
+
+\begin{lstlisting}[language=Scala,numbers=none]
SEQ(CHAR('f'), SEQ(CHAR('o'), CHAR('r')))
-\end{alltt}
-\end{quote}
+\end{lstlisting}
+
\noindent This gets quickly unreadable when the strings and
regular expressions get more complicated. In other functional
programming language, you can explicitly write a conversion
-function that takes a string, say {\tt for}, and generates the
+function that takes a string, say \code{for}, and generates the
regular expression above. But then your code is littered with
such conversion function.
In Scala you can do better by ``hiding'' the conversion
-functions. The keyword for doing this is {\tt implicit}.
+functions. The keyword for doing this is \code{implicit}.
Consider the code
-\begin{quote}
+
\begin{lstlisting}[language=Scala]
import scala.language.implicitConversions
@@ -792,30 +789,30 @@
implicit def string2rexp(s: String) : Rexp =
charlist2rexp(s.toList)
\end{lstlisting}
-\end{quote}
+
\noindent where the first seven lines implement a function
that given a list of characters generates the corresponding
regular expression. In Lines 9 and 10, this function is used
for transforming a string into a regular expression. Since the
-{\tt string2rexp}-function is declared as {\tt implicit} the
+\code{string2rexp}-function is declared as \code{implicit} the
effect will be that whenever Scala expects a regular
expression, but I only give it a string, it will automatically
-insert a call to the {\tt string2rexp}-function. I can now
+insert a call to the \code{string2rexp}-function. I can now
write for example
-\begin{quote}
-\begin{alltt}
+
+\begin{lstlisting}[language=Scala,numbers=none]
scala> ALT("ab", "ac")
res9: ALT = ALT(SEQ(CHAR(a),CHAR(b)),SEQ(CHAR(a),CHAR(c)))
-\end{alltt}
-\end{quote}
+\end{lstlisting}
+
Using implicit definitions, Scala allows me to introduce
some further syntactic sugar for regular expressions:
-\begin{quote}
+
\begin{lstlisting}[language=Scala]
implicit def RexpOps(r: Rexp) = new {
def | (s: Rexp) = ALT(r, s)
@@ -831,18 +828,18 @@
def % = STAR(s)
}
\end{lstlisting}
-\end{quote}
+
\noindent This might seem a bit overly complicated, but its effect is
that I can now write regular expressions such as $ab + ac$
even simpler as
-\begin{quote}
-\begin{alltt}
+
+\begin{lstlisting}[language=Scala,numbers=none]
scala> "ab" | "ac"
res10: ALT = ALT(SEQ(CHAR(a),CHAR(b)),SEQ(CHAR(a),CHAR(c)))
-\end{alltt}
-\end{quote}
+\end{lstlisting}
+
\noindent I leave you to figure out what the other
syntactic sugar in the code above stands for.
@@ -854,19 +851,19 @@
context of regular expressions this feature comes in handy:
Say you are fed up with writing many alternatives as
-\begin{quote}
-\begin{alltt}
+
+\begin{lstlisting}[language=Scala,numbers=none]
ALT(..., ALT(..., ALT(..., ...)))
-\end{alltt}
-\end{quote}
+\end{lstlisting}
+
\noindent To make it difficult, you do not know how deep such
alternatives are nested. So you need something flexible that
can take as many alternatives as needed. In Scala one can
-achieve this by adding a {\tt *} to the type of an argument.
+achieve this by adding a \code{*} to the type of an argument.
Consider the code
-\begin{quote}
+
\begin{lstlisting}[language=Scala]
def Alts(rs: List[Rexp]) : Rexp = rs match {
case Nil => NULL
@@ -876,7 +873,7 @@
def ALTS(rs: Rexp*) = Alts(rs.toList)
\end{lstlisting}
-\end{quote}
+
\noindent The function in Lines 1 to 5 takes a list of regular
expressions and converts it into an appropriate alternative
@@ -885,11 +882,11 @@
effect of this code is that I can write the regular
expression for keywords as
-\begin{quote}
-\begin{alltt}
+
+\begin{lstlisting}[language=Scala,numbers=none]
ALTS("for", "def", "yield", "implicit", "if", "match", "case")
-\end{alltt}
-\end{quote}
+\end{lstlisting}
+
\noindent Again I leave you to it to find out how much this
simplifies the regular expression in comparison if I had to
@@ -911,20 +908,20 @@
While I am quite enthusiastic about Scala, I am also happy to
admit that it has more than its fair share of faults. The
problem seen earlier of having to give an explicit type to
-{\tt toSet}, but not {\tt toList} is one of them. There are
+\code{toSet}, but not \code{toList} is one of them. There are
also many ``deep'' ideas about types in Scala, which even to
me as seasoned functional programmer are puzzling. Whilst
implicits are great, they can also be a source of great
headaches, for example consider the code:
-\begin{quote}
-\begin{alltt}
+
+\begin{lstlisting}[language=Scala,numbers=none]
scala> List (1, 2, 3) contains "your mom"
res1: Boolean = false
-\end{alltt}
-\end{quote}
+\end{lstlisting}
-\noindent Rather than returning {\tt false}, this code should
+
+\noindent Rather than returning \code{false}, this code should
throw a typing-error. There are also many limitations Scala
inherited from the JVM that can be really annoying. For
example a fixed stack size.