--- a/handouts/scala-ho.tex Tue Aug 26 17:26:06 2014 +0100
+++ b/handouts/scala-ho.tex Wed Aug 27 16:11:32 2014 +0100
@@ -6,7 +6,8 @@
\usepackage{amsmath}
\usepackage{../langs}
\usepackage{mathpazo}
-\usepackage[scaled=.9]{helvet}
+\usepackage{marvosym}
+%%%\usepackage[scaled=.95]{helvet}
\newcommand{\dn}{\stackrel{\mbox{\scriptsize def}}{=}}%
\definecolor{codegray}{gray}{0.9}
@@ -16,23 +17,25 @@
\section*{A Crash-Course on Scala}
-Scala is programming language that combines functional and
+Scala is a programming language that combines functional and
object-oriented programming-styles, and has received in the
last five years quite a bit of attention. One reason for this
attention is that, like the Java programming language, Scala
compiles to the Java Virtual Machine (JVM) and therefore can
run under MacOSX, Linux and Windows.\footnote{There are also
experimental backends for Android and JavaScript.} Unlike
-Java, however, Scala often allows programmers to write concise
-and elegant code. Some therefore say Scala is the much better
-Java. If you want to try it out yourself, the Scala compiler
-can be downloaded from
+Java, however, Scala often allows programmers to write very
+concise and elegant code. Some therefore say Scala is the much
+better Java. The Guardian, Twitter, Coursera, LinkedIn to name
+a few either rely entirely in their infrastructures on Scala,
+or some parts of there infrastructure uses it. If you want to
+try it out yourself, the Scala compiler can be downloaded from
\begin{quote}
\url{http://www.scala-lang.org}
\end{quote}
-Why do I use Scala in the AFL course? Actually, you can do
+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
@@ -40,20 +43,21 @@
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, ML,
-F\# and so on.
+into any other functional language, for example Haskell,
+Standard ML, F\#, Ocaml and so on.
-Writing programs in Scala can be done with the Eclipse IDE and
-also with IntelliJ, 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
+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
+\begin{quote}
\begin{alltt}
$ scala\small
Welcome to Scala version 2.11.2 (Java HotSpot(TM) 64-Bit Server VM).
@@ -62,26 +66,31 @@
scala>
\end{alltt}
+\end{quote}
\noindent The precise response may vary due to the platform
-where you installed Scala. At the scala prompt you can type
+where you installed Scala. At the Scala prompt you can type
things like {\tt 2 + 3} \keys{Ret} and the output will be
+\begin{quote}
\begin{alltt}
scala> 2 + 3
res0: Int = 5
\end{alltt}
+\end{quote}
\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
+\begin{quote}
\begin{alltt}
scala> print ("hello world")
hello world
\end{alltt}
+\end{quote}
-\noindent Note that in this case there is no result: the
+\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
@@ -89,14 +98,14 @@
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
-point in due course, but if you are curious now, the latter
-kind of functions always have as return type {\tt Unit}.
+point later, but if you are curious now, the latter kind of
+functions always have as return type {\tt Unit}.
-If you want to write a stand-alone app, you can implement
-an object that is an instance of {\tt App}, say
+If you want to write a stand-alone app in Scala, you can
+implement an object that is an instance of {\tt App}, say
\begin{quote}
-\begin{lstlisting}[language=Scala]
+\begin{lstlisting}[language=Scala,numbers=none]
object Hello extends App {
println ("hello world")
}
@@ -106,23 +115,27 @@
\noindent save it in a file, say {\tt hellow-world.scala}, and
then run the compiler and runtime environment:
+\begin{quote}
\begin{alltt}
$ scalac hello-world.scala
$ scala Hello
hello world
\end{alltt}
+\end{quote}
-As mentioned above, Scala targets the JVM and
-consequently Scala programs can also be executed by the
-bog-standard Java runtime. This only requires the inclusion of
-{\tt scala-library.jar}, which on my computer can be done as
+As mentioned above, Scala targets the JVM and consequently
+Scala programs can also be executed by the bog-standard Java
+Runtime. This only requires the inclusion of {\tt
+scala-library.jar}, which on my computer can be done as
follows:
+\begin{quote}
\begin{alltt}
$ scalac hello-world.scala
$ java -cp /usr/local/src/scala/lib/scala-library.jar:. Hello
hello world
\end{alltt}
+\end{quote}
\subsection*{Inductive Datatypes}
@@ -148,23 +161,25 @@
inner nodes---sequence, alternative and star---and three kinds
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! ;o)}
+define this kind of inductive datatypes in Java\footnote{Happy
+programming! \Smiley} and then compare it how
+it can be defined in Scala.
Implementing the regular expressions from above in Scala is
-very simple: It first requires an \emph{abstract class}, say,
-{\tt Rexp}. This will act as the type for regular expressions.
-Second, it requires some instances. 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
-code is as follows:
+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
+follows:
\begin{quote}
-\begin{lstlisting}[language=Scala]
+\begin{lstlisting}[language=Scala,numbers=none]
abstract class Rexp
case object NULL extends Rexp
case object EMPTY extends Rexp
@@ -178,60 +193,69 @@
\noindent In order to be an instance of {\tt Rexp}, each case
object and case class needs to extend {\tt Rexp}. Given the
grammar above, I hope you can see the underlying pattern. If
-you want to play further with such definitions, feel free to
-define for example binary trees.
+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. If you want to assign
-this regular expression to a variable, you can use the keyword
-{\tt val} and type
+{\tt '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
+\begin{quote}
\begin{alltt}
scala> val r = ALT(CHAR('a'), CHAR('b'))
r: ALT = ALT(CHAR(a),CHAR(b))
\end{alltt}
+\end{quote}
\noindent As you can see, in order to make such assignments,
no constructor is required in the class (as in Java). However,
if there is the need for some non-standard initialisation, you
-can of course define such a constructor in Scala. But we omit
-this here.
+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
-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
+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
+\begin{quote}
\begin{alltt}
scala> val ls = List(ALT(CHAR('a'), CHAR('b')), NULL)
ls: List[Rexp] = List(ALT(CHAR(a),CHAR(b)), NULL)
\end{alltt}
+\end{quote}
-\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 type is {\tt 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.}
+\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
+type in this example, you will notice that the type contains
+some further information, but lets ignore this for the
+moment.}
-For types like {\tt List[...]} the general rule is that when a
-type takes another type as argument, then this is written in
-angle-brackets. This can also contain nested types as in {\tt
-List[Set[Rexp]]}, which is a list of sets each of which
-contains regular expressions.
+For compound types like {\tt 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
+list of sets each of which contains regular expressions.
\subsection*{Functions and Pattern-Matching}
I mentioned above that Scala is a very elegant programming
language for the code we will write in this module. This
-elegance mainly stems from the fact that functions can be
-implemented very easily in Scala. Lets first consider a
+elegance mainly stems from the fact that in addition to
+inductive datatypes, also functions can be implemented very
+easily in Scala. To show you this, lets first consider a
problem from number theory, called the \emph{Collatz-series},
which corresponds to a famous unsolved problem in
mathematics.\footnote{See for example
@@ -256,7 +280,7 @@
this as the following function in Scala:
\begin{quote}
-\lstinputlisting[language=Scala]{../progs/collatz.scala}
+\lstinputlisting[language=Scala,numbers=none]{../progs/collatz.scala}
\end{quote}
\noindent The keyword for function definitions is {\tt def}
@@ -264,37 +288,40 @@
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 for arbitrary precision integers.
-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 =} comes
-the \emph{body} of the function implementing what the function
-is supposed to do. What the {\tt 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.
+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 =}
+comes the \emph{body} of the function implementing what the
+function is supposed to do. What the {\tt 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.
Notice a quirk in Scala's syntax for {\tt 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
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
if there are many cases. If we wanted to define a function
-over regular expressions in say Java, which does not have
-pattern-matching, the resulting code would just be awkward.
+over regular expressions in Java, for example, which does not
+have pattern-matching, the resulting code would be just
+awkward.
-Mathematicians already use the power of pattern-matching, for
-example, when they define the function that takes a regular
-expression and produces another regular expression that can
-recognise the reversed strings. The resulting recursive
-function is often defined as follows:
+Mathematicians already use the power of pattern-matching when
+they define the function that takes a regular expression and
+produces another regular expression that can recognise the
+reversed strings. The resulting recursive function is often
+defined as follows:
\begin{center}
-\begin{tabular}{r@{\hspace{1mm}}c@{\hspace{1mm}}l}
+\begin{tabular}{r@{\hspace{2mm}}c@{\hspace{2mm}}l}
$rev(\varnothing)$ & $\dn$ & $\varnothing$\\
$rev(\epsilon)$ & $\dn$ & $\epsilon$\\
$rev(c)$ & $\dn$ & $c$\\
@@ -304,8 +331,10 @@
\end{tabular}
\end{center}
-\noindent The corresponding Scala code looks very similar
-to this definition, thanks to pattern-matching.
+\noindent This function is defined by recursion analysing each
+pattern of what the regular expression could look like. The
+corresponding Scala code looks very similar to this
+definition, thanks to pattern-matching.
\begin{quote}
@@ -314,26 +343,26 @@
\noindent The keyword for starting a pattern-match is {\tt
match} followed by a list of {\tt case}s. Before the match
-can be another pattern, but often as in the case above,
-it is just a variable you want to pattern-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).
-In the {\tt rev}-function above, each case 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.
+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.
-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 the result $ba + ca$, which
-can recognise the reversed strings $ba$ and $ca$.
+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
+strings $ba$ and $ca$.
In Scala each pattern-match can also be guarded as in
@@ -365,52 +394,58 @@
build the list of squares. The list of numbers from 1 to 10
can be constructed in Scala as follows:
+\begin{quote}
\begin{alltt}
scala> (1 to 10).toList
res1: List[Int] = List(1, 2, 3, 4, 5, 6, 7, 8, 9, 10)
\end{alltt}
+\end{quote}
\noindent Generating from this list the list of squares in a
-non-functional 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.
+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.
Maps essentially take a function that describes how each
element is transformed (for example squaring) and a list over
which this function should work. There are two forms to
-express maps in Scala. The first way is in a {\tt
+express such maps in Scala. The first way is in a {\tt
for}-construction. Squaring the numbers from 1 to 10 would
look in this form as follows:
+\begin{quote}
\begin{alltt}
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}
\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}. In consequence we specified the list where each {\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 by using directly {\tt map} as follows:
+element needs to be transformed. This can also be expressed in
+a second way in Scala by using directly {\tt map} as follows:
+\begin{quote}
\begin{alltt}
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
+\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 on 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 them into
-equivalent maps.
+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.
The very charming feature of Scala is that such maps or {\tt
@@ -419,32 +454,38 @@
example if we instead compute the reminders modulo $3$ of this
list, we can write
+\begin{quote}
\begin{alltt}
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}
\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
+\begin{quote}
\begin{alltt}
scala> (1 to 10).toSet[Int].map(n => n \% 3)
res5 = Set(2, 1, 0)
\end{alltt}
+\end{quote}
\noindent This is the correct result for sets, as there are
-only 3 equivalence classes of integers modulo 3. Note that we
-need to ``help'' Scala in this example to transform the
+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}-construction are just
-syntactic variants of each other, the latter can also be
+type {\tt Int}. Since maps and {\tt for}-constructions 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
res5 = Set(2, 1, 0)
\end{alltt}
+\end{quote}
While hopefully this all looks reasonable, there is one
complication: In the examples above we always wanted to
@@ -453,10 +494,12 @@
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
-guessed it, does not produce any result, but only produces, in
-the functional-programming-lingo, a side-effect. Printing out
-the list of numbers from 1 to 5 would look as follows
+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}
scala> for (n <- (1 to 5).toList) println(n)
1
@@ -465,11 +508,13 @@
4
5
\end{alltt}
+\end{quote}
\noindent
where you need to omit the keyword {\tt yield}. You can
also do more elaborate calculations such as
+\begin{quote}
\begin{alltt}
scala> for (n <- (1 to 5).toList) \{
val square_n = n * n
@@ -481,18 +526,19 @@
4 * 4 = 16
5 * 5 = 25
\end{alltt}
+\end{quote}
-\noindent
-In this code I use a \emph{string interpolation},
-written {\tt s"..."} in order to print out an equation.
-This string interpolation allows me to refer to the integer
-values {\tt n} and {\tt square\_n} inside a string. This
-is very convenient for printing out ``things''.
+\noindent In this code I use a variable assignment and a
+\emph{string interpolation}, written {\tt 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
+a string. This is very convenient for printing out ``things''.
The corresponding map construction for functions with
-side-effexts is in Scala called {\tt foreach}. So you
+side-effects is in Scala called {\tt foreach}. So you
could also write
+\begin{quote}
\begin{alltt}
scala> (1 to 5).toList.foreach(n => println(n))
1
@@ -501,9 +547,11 @@
4
5
\end{alltt}
+\end{quote}
\noindent or even just
+\begin{quote}
\begin{alltt}
scala> (1 to 5).toList.foreach(println)
1
@@ -512,12 +560,18 @@
4
5
\end{alltt}
+\end{quote}
-\noindent If you want to find out more maps and functions
-with side-efffects, 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}, but
-then reacts with an interesting result.
+\noindent Again I hope this reminds you a bit of your
+lambda-calculus lessons, where an explanation is given why
+both forms produce the same result.
+
+
+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
+functions, but then reacts with a slightly interesting result.
\subsection*{Types}
@@ -531,7 +585,7 @@
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 compicated
+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
@@ -544,31 +598,35 @@
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,
-which need to be given in angle-brackets, for example {\tt
-List[Int]} or {\tt Set[List[String]]} or {\tt Map[Int, Int]}.
+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]}.
-There are e few special type-constructors. One is for tuples,
-which is written with parentheses. For example {\tt (Int, Int,
-String)} for a triple consisting of two integers and a string.
-Tuples are helpful if you want to define a function with
-multiple results, say the function returning the quotient and
-reminder of two numbers. For this you might define:
+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
+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{alltt}
-def quo_rem(m: Int, n: Int) : (Int, Int) = (m \verb|/| n, m \% n)
-\end{alltt}
+\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)}.
Another special type-constructor is for functions, written
-{\tt =>}. For example, the type {\tt Int => String} stands
-for a function that takes an integer as argument and produces
-a string. A function of this type is for instance
+as the arrow {\tt =>}. For example, the type {\tt 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]
+\begin{lstlisting}[language=Scala,numbers=none]
def mk_string(n: Int) : String = n match {
case 0 => "zero"
case 1 => "one"
@@ -579,32 +637,269 @@
\end{quote}
\noindent Unlike other functional programming languages, there
-is no easy way to find out the types of existing functions
-except for looking into the documentation
+is in Scala no easy way to find out the types of existing
+functions, except by looking into the documentation
\begin{quote}
\url{http://www.scala-lang.org/api/current/}
\end{quote}
-\noindent The function arrow can also be iterated, as in {\tt
+The function arrow can also be iterated, as in {\tt
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 is
+function of this type would be
\begin{quote}
-\begin{lstlisting}[language=Scala]
+\begin{lstlisting}[language=Scala,numbers=none]
def chk_string(n: Int, s: String) : Boolean =
mk_string(n) == s
\end{lstlisting}
\end{quote}
-\noindent
+\noindent which checks whether the integer {\tt n} corresponds
+to the name {\tt s} given by the function {\tt mk\_string}.
+
+Coming back to the type {\tt 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).
+
+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
+print out strings, but the latter adds a line break. You can
+call {\tt foreach} with either of them and thus changing how,
+for example, five numbers are printed.
+
+\begin{quote}
+\begin{alltt}
+scala> (1 to 5).toList.foreach(print)
+12345
+scala> (1 to 5).toList.foreach(println)
+1
+2
+3
+4
+5
+\end{alltt}
+\end{quote}
+
+\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,
+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
+any kind of function (not just functions that, for example,
+take as input an integer and produce a string like above).
+This means we cannot fix the type of the generic traversal
+functions, but have to keep them
+\emph{polymorphic}.\footnote{Another interestic topic about
+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,
+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
+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
+that a function does not have a result, but potentially causes
+some side-effect. Typical examples are the printing functions,
+like {\tt print}.
+
\subsection*{Cool Stuff}
+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
+more sophisticated, namely only returns the first 10000
+characters of a webpage in case a ``webpage'' is too large.
+Why is that code-snippet of any interest? Well, try
+implementing reading from a webpage in Java. I also like the
+possibility of triple-quoting strings, which I have only seen
+in Scala so far. The idea behind this is that in such a string
+all characters are interpreted literally---there are no
+escaped characters, like \verb|\n| for newlines.
+
+My second wow-moment I had with a feature of Scala that other
+functional programming languages do not have. This feature is
+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}:
+
+\begin{quote}
+\begin{alltt}
+SEQ(CHAR('f'), SEQ(CHAR('o'), CHAR('r')))
+\end{alltt}
+\end{quote}
+
+\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
+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}.
+Consider the code
+
+\begin{quote}
+\begin{lstlisting}[language=Scala]
+import scala.language.implicitConversions
+
+def charlist2rexp(s: List[Char]) : Rexp = s match {
+ case Nil => EMPTY
+ case c::Nil => CHAR(c)
+ case c::s => SEQ(CHAR(c), charlist2rexp(s))
+}
+
+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
+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
+write for example
+
+\begin{quote}
+\begin{alltt}
+scala> ALT("ab", "ac")
+res9: ALT = ALT(SEQ(CHAR(a),CHAR(b)),SEQ(CHAR(a),CHAR(c)))
+\end{alltt}
+\end{quote}
+
+
+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)
+ def ~ (s: Rexp) = SEQ(r, s)
+ def % = STAR(r)
+}
+
+implicit def stringOps(s: String) = new {
+ def | (r: Rexp) = ALT(s, r)
+ def | (r: String) = ALT(s, r)
+ def ~ (r: Rexp) = SEQ(s, r)
+ def ~ (r: String) = SEQ(s, r)
+ def % = STAR(s)
+
+}
+\end{lstlisting}
+\end{quote}
+
+\noindent This might seem a bit complicated, but its effect is
+that I can now write regular expressions such as $ab + ac$
+even simpler as
+
+\begin{quote}
+\begin{alltt}
+scala> "ab" | "ac"
+res10: ALT = ALT(SEQ(CHAR(a),CHAR(b)),SEQ(CHAR(a),CHAR(c)))
+\end{alltt}
+\end{quote}
+
+\noindent I leave you to figure out what the other
+syntactic sugar in the code above stands for.
+
+One more useful feature of Scala is the ability to define
+functions with variable argument lists. This is a feature that
+is already present in old languages, like C, but seems to have
+been forgotten in the meantime---Java does not have it. In the
+context of regular expressions this feature comes in handy:
+Say you are fed up with writing many alternatives as
+
+\begin{quote}
+\begin{alltt}
+ALT(..., ALT(..., ALT(..., ...)))
+\end{alltt}
+\end{quote}
+
+\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.
+Consider the code
+
+\begin{quote}
+\begin{lstlisting}[language=Scala]
+def Alts(rs: List[Rexp]) : Rexp = rs match {
+ case Nil => NULL
+ case r::Nil => r
+ case r::rs => ALT(r, Alts(rs))
+}
+
+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
+regular expression. In Line 7 there is a wrapper for this
+function which uses the feature of varying argument lists. The
+effect of this code is that I can write the regular
+expression for keywords as
+
+\begin{quote}
+\begin{alltt}
+ALTS("for", "def", "yield", "implicit", "if", "match", "case")
+\end{alltt}
+\end{quote}
+
+\noindent Again I leave you to it how much this simplifies the
+regular expression in comparison if I had to write this by
+hand using only the ``plain'' regular expressions from the
+inductive datatype.
+
\subsection*{More Info}
+
+
\end{document}
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