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-\documentclass{article}
-\usepackage{../style}
-\usepackage{../langs}
-\usepackage{marvosym}
-
-%cheat sheet
-%http://worldline.github.io/scala-cheatsheet/
-
-\begin{document}
-
-\section*{A Crash-Course on Scala}
-
-Scala is a programming language that combines functional and
-object-oriented programming-styles. It has received quite a
-bit of attention in the last five years or so. One reason for
-this attention is that, like the Java programming language,
-Scala compiles to the Java Virtual Machine (JVM) and therefore
-Scala programs can run under MacOSX, Linux and
-Windows.\footnote{There are also experimental backends for
-Android and JavaScript; and also work is under way to have a
-native compiler, see \url{https://github.com/scala-native/scala-native}.} Unlike Java, however, Scala often
-allows programmers to write very concise and elegant code.
-Some therefore say: Scala is the much better Java. A number of
-companies, The Guardian, Twitter, Coursera, FourSquare,
-LinkedIn to name a few, either use Scala exclusively in
-production code, or at least to some substantial degree. It
-also seems to be useful in job-interviews (in Data Science)
-according to this annectotical report
-
-\begin{quote}
-\url{https://techcrunch.com/2016/06/14/scala-is-the-new-golden-child/}
-\end{quote}
-
-\noindent
-If you want to try out Scala yourself, the official Scala compiler can be
-downloaded from
-
-\begin{quote}
-\url{http://www.scala-lang.org}
-\end{quote}
-
-\noindent
-A ready-made bundle with the Eclipse IDE is at
-
-\begin{quote}
-\url{http://scala-ide.org/download/sdk.html}
-\end{quote}
-
-Why do I use Scala in the AFL module? Actually, you can do
-\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, I would first have
-to go through heaps of boilerplate code and the code-snippets
-would not look pretty. 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, Swift,
-Standard ML, F$^\#$, Ocaml and so on.
-
-Developing programs in Scala can be done with the Eclipse IDE
-and also with the 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
-\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{lstlisting}[language={},numbers=none,basicstyle=\ttfamily\small]
-$ scala
-Welcome to Scala version 2.11.8 (Java HotSpot(TM) 64-Bit Server VM).
-Type in expressions for evaluation. Or try :help.
-
-scala>
-\end{lstlisting}
-
-\noindent Of course the precise response may vary due to the
-version and platform where you installed Scala. At the Scala
-prompt you can type things like \code{2 + 3} \keys{Ret} and
-the output will be
-
-\begin{lstlisting}[numbers=none]
-scala> 2 + 3
-res0: Int = 5
-\end{lstlisting}
-
-\noindent indicating that the result of the addition is of
-type \code{Int} and the actual result is 5. Another classic
-example you can try out is
-
-\begin{lstlisting}[numbers=none]
-scala> print("hello world")
-hello world
-\end{lstlisting}
-
-\noindent Note that in this case there is no result. The
-reason is that \code{print} does not actually produce a result
-(there is no \code{resXX} and no type), 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 has \code{Unit} as
-return type.
-
-If you want to write a stand-alone app in Scala, you can
-implement an object that is an instance of \code{App}, say
-
-\begin{lstlisting}[numbers=none]
-object Hello extends App {
-    println("hello world")
-}
-\end{lstlisting}
-
-\noindent save it in a file, say {\tt hello-world.scala}, and
-then run the compiler and runtime environment:
-
-\begin{lstlisting}[language={},numbers=none,basicstyle=\ttfamily\small]
-$ scalac hello-world.scala
-$ scala Hello
-hello world
-\end{lstlisting}
-
-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{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{lstlisting}
-
-\noindent You might need to adapt the path to where you have
-installed Scala.
-
-\subsection*{Inductive Datatypes}
-
-The elegance and conciseness of Scala programs are often a
-result of inductive datatypes that can be easily defined in
-Scala. For example in ``every-day mathematics'' we define
-regular expressions simply by giving the grammar
-
-\begin{center}
-\begin{tabular}{r@{\hspace{2mm}}r@{\hspace{2mm}}l@{\hspace{13mm}}l}
-  $r$ & $::=$ &   $\ZERO$            & null\\
-        & $\mid$ & $\ONE$            & empty string\\
-        & $\mid$ & $c$               & single character\\
-        & $\mid$ & $r_1 \cdot r_2$   & sequence\\
-        & $\mid$ & $r_1 + r_2$       & alternative / choice\\
-        & $\mid$ & $r^\star$             & star (zero or more)\\
-  \end{tabular}
-\end{center}
-
-\noindent This grammar specifies what regular expressions are
-(essentially a kind of tree-structure with three kinds of
-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! \Smiley} and then compare it with how it can be
-implemented in Scala.
-
-Implementing the regular expressions from above in Scala is
-actually very simple: It first requires an \emph{abstract
-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 $\ZERO$ and $\ONE$ 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}, whereas the ones with arguments are
-\emph{case classes}. The corresponding Scala code is as
-follows:
-
-\begin{lstlisting}[numbers=none]
-abstract class Rexp 
-case object ZERO extends Rexp
-case object ONE extends Rexp
-case class CHAR (c: Char) extends Rexp
-case class SEQ (r1: Rexp, r2: Rexp) extends Rexp 
-case class ALT (r1: Rexp, r2: Rexp) extends Rexp 
-case class STAR (r: Rexp) extends Rexp 
-\end{lstlisting}
-
-\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 in Scala, you
-can represent the regular expression for $a + b$, for example,
-as \code{ALT(CHAR('a'), CHAR('b'))}. Expressions such as
-\code{'a'} stand for ASCII characters, though in the output
-syntax, as you can see below, the quotes are omitted. In a
-later section we will see how we can support the more
-mathematical infix notation for regular expression operators
-in Scala. If you want to assign this regular expression to a
-variable, you can use the keyword \code{val} and type
-
-\begin{lstlisting}[numbers=none]
-scala> val r = ALT(CHAR('a'), CHAR('b'))
-r: ALT = ALT(CHAR(a),CHAR(b))
-\end{lstlisting}
-
-\noindent As you can see, in order to make such assignments,
-no \code{new} or 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 too. But we omit such ``tricks'' here. 
-
-Note that Scala in its response says the variable \code{r} is
-of type \code{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
-\code{ALT}, since it is the abstract class for all regular
-expressions. But in this particular 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{lstlisting}[numbers=none]
-scala> val ls = List(ALT(CHAR('a'), CHAR('b')), ZERO)
-ls: List[Rexp] = List(ALT(CHAR(a),CHAR(b)), ZERO)
-\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 \code{Rexp}.\footnote{If you
-type in this example, you will notice that the type contains
-some further information, but let us ignore this for the
-moment.} 
-
-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 \code{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 in addition to
-inductive datatypes, also functions can be implemented very
-easily in Scala. To show you this, let us 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
-\url{http://mathworld.wolfram.com/CollatzProblem.html}.}
-Mathematicians define this series as:
-
-\[
-collatz_{n + 1} \dn 
-\begin{cases}
-\frac{1}{2} * collatz_n & \text{if $collatz_n$ is even}\\
-3 * collatz_n + 1 & \text{if $collatz_n$ is odd}
-\end{cases}
-\]
-
-\noindent The famous unsolved question is whether this
-series started with any $n > 0$ as $collatz_0$ will always
-return to $1$. This is obvious when started with $1$, and also
-with $2$, but already needs a bit of head-scratching for the
-case of $3$.
-
-If we want to avoid the head-scratching, we could implement
-this as the following function in Scala:
-
-\lstinputlisting[numbers=none]{../progs/collatz.scala}
-
-\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 to be
-annotated. 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 1. Finally, after the
-\code{=} comes the \emph{body} of the function implementing
-what the function is supposed to do. What the \code{collatz}
-function does should be pretty self-explanatory: the function
-first tests whether \code{n} is equal to 1 in which case it
-returns \code{true} and so on.
-
-Notice the 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 \code{then} keyword in
-Scala.
-
-The real power of Scala comes, however, from the ability to
-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 just be
-awkward.
-
-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. They define this function as follows:
-
-\begin{center}
-\begin{tabular}{r@{\hspace{2mm}}c@{\hspace{2mm}}l}
-$rev(\ZERO)$   & $\dn$ & $\ZERO$\\
-$rev(\ONE)$      & $\dn$ & $\ONE$\\
-$rev(c)$             & $\dn$ & $c$\\
-$rev(r_1 + r_2)$     & $\dn$ & $rev(r_1) + rev(r_2)$\\
-$rev(r_1 \cdot r_2)$ & $\dn$ & $rev(r_2) \cdot rev(r_1)$\\
-$rev(r^*)$                   & $\dn$ & $rev(r)^*$\\
-\end{tabular}
-\end{center}
-
-\noindent It is defined by recursion analysing each pattern of
-what the regular expression might look like. The corresponding
-Scala code looks very similar to this definition, thanks to
-pattern-matching.
-
-\lstinputlisting[language=Scala]{../progs/rev.scala}
-
-\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 \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
-\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 
-\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{lstlisting}[ numbers=none]
-case Pattern if Condition => Do_Something
-\end{lstlisting}
-
-\noindent This allows us, for example, to re-write the 
-\code{collatz}-function from above as follows:
- 
-\lstinputlisting[language=Scala]{../progs/collatz2.scala}
-
-
-\noindent Although in this particular case the pattern-match
-does not improve the code in any way. In cases like
-\code{rev}, however, it is really crucial. The underscore in
-Line 4 indicates that we do not care what the pattern looks
-like. Thus this case acts like a default case whenever the
-cases above did not match. Cases are always tried out from top
-to bottom.
- 
-\subsection*{Loops, or better the Absence thereof}
-
-Coming from Java or C, you might be surprised that Scala does
-not really have loops. It has instead, what is in functional
-programming called, \emph{maps}. To illustrate how they work,
-let us assume you have a list of numbers from 1 to 8 and want to
-build the list of squares. The list of numbers from 1 to 8 
-can be constructed in Scala as follows:
-
-\begin{lstlisting}[numbers=none]
-scala> (1 to 8).toList
-res1: List[Int] = List(1, 2, 3, 4, 5, 6, 7, 8)
-\end{lstlisting}
-
-\noindent Generating from this list, the list of squares in a
-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. 
-
-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 called a
-\emph{for-comprehension}. Squaring the numbers from 1 to 8
-would look in this form as follows:
-
-\begin{lstlisting}[numbers=none]
-scala> for (n <- (1 to 8).toList) yield n * n
-res2: List[Int] = List(1, 4, 9, 16, 25, 36, 49, 64)
-\end{lstlisting}
-
-\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 8).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{lstlisting}[numbers=none]
-scala> (1 to 8).toList.map(n => n * n)
-res3 = List(1, 4, 9, 16, 25, 36, 49, 64)
-\end{lstlisting}
-
-\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).
-
-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}[numbers=none]
-scala> (1 to 8).toList.map(n => n % 3)
-res4 = List(1, 2, 0, 1, 2, 0, 1, 2)
-\end{lstlisting}
-
-\noindent If we, however, transform the numbers 1 to 8 not
-into a list, but into a set, and then compute the reminders
-modulo 3 we obtain
-
-\begin{lstlisting}[numbers=none]
-scala> (1 to 8).toSet[Int].map(n => n % 3)
-res5 = Set(2, 1, 0)
-\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 \code{Int}. Since maps and for-comprehensions are
-just syntactic variants of each other, the latter can also be
-written as
-
-\begin{lstlisting}[numbers=none]
-scala> for (n <- (1 to 8).toSet[Int]) yield n % 3
-res5 = Set(2, 1, 0)
-\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}[numbers=none]
-scala> for (n <- (1 to 4).toList; 
-            m <- ('a' to 'c').toList) yield (n, m)
-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}[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 in the for-comprehension
-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 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{lstlisting}[numbers=none]
-scala> for (n <- (1 to 5).toList) print(n)
-12345
-\end{lstlisting}
-
-\noindent
-where you need to omit the keyword \code{yield}. You can
-also do more elaborate calculations such as
-
-\begin{lstlisting}[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{lstlisting}
-
-\noindent In this code I use a variable assignment (\code{val
-square_n = ...} ) and also what is called in Scala a
-\emph{string interpolation}, written \code{s"..."}. The latter
-is for printing out an equation. It allows me to 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 \code{foreach}. So you 
-could also write
-
-
-\begin{lstlisting}[numbers=none]
-scala> (1 to 5).toList.foreach(n => print(n))
-12345
-\end{lstlisting}
-
-
-\noindent or even just
-
-\begin{lstlisting}[numbers=none]
-scala> (1 to 5).toList.foreach(print)
-12345
-\end{lstlisting}
-
-\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 \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 \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 \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 \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 
-
-\begin{lstlisting}[ numbers=none]
-(Int, Int, String)
-\end{lstlisting}
-
-\noindent is for a triple (a tuple with three components---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{lstlisting}[ numbers=none]
-def quo_rem(m: Int, n: Int) : (Int, Int) = (m / n, m % n)
-\end{lstlisting}
-
-
-\noindent Since this function returns a pair of integers, its
-return type needs to be of type \code{(Int, Int)}.
-Incidentally, this is also the input type of this function.
-Notice this function takes \emph{two} arguments, namely
-\code{m} and \code{n}, both of which are integers. They are
-``packaged'' in a pair. Consequently the complete type of
-\code{quo_rem} is
-
-\begin{lstlisting}[ numbers=none]
-(Int, Int) => (Int, Int)
-\end{lstlisting}
-
-Another special type-constructor is for functions, written as
-the arrow \code{=>}. For example, the type \code{Int =>
-String} is for a function that takes an integer as input
-argument and produces a string as result. A function of this
-type is for instance
-
-\begin{lstlisting}[numbers=none]
-def mk_string(n: Int) : String = n match {
-  case 0 => "zero"
-  case 1 => "one"
-  case 2 => "two"
-  case _ => "many" 
-} 
-\end{lstlisting}
-
-\noindent It takes an integer as input argument and returns a
-string. Unlike other functional programming languages, there
-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}
-
-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{lstlisting}[numbers=none]
-def chk_string(n: Int)(s: String) : Boolean = 
-  mk_string(n) == s
-\end{lstlisting}
-
-
-\noindent which checks whether the integer \code{n}
-corresponds to the name \code{s} given by the function
-\code{mk\_string}. Notice the unusual way of specifying the
-arguments of this function: the arguments are given one after
-the other, instead of being in a pair (what would be the type
-of this function then?). This way of specifying the arguments
-can be useful, for example in situations like this
-
-\begin{lstlisting}[numbers=none]
-scala> List("one", "two", "three", "many").map(chk_string(2))
-res4 = List(false, true, false, false)
-
-scala> List("one", "two", "three", "many").map(chk_string(3))
-res5 = List(false, false, false, true)
-\end{lstlisting}
-
-\noindent In each case we can give to \code{map} a specialised
-version of \code{chk_string}---once specialised to 2 and once
-to 3. This kind of ``specialising'' a function is called
-\emph{partial application}---we have not yet given to this
-function all arguments it needs, but only some of them.
-
-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 their type is (in this case two arguments,
-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). Remember that \code{mk_string} is just 
-such a function. So how can we use it? For this define
-the somewhat silly function \code{apply_3}:
-
-\begin{lstlisting}[numbers=none]
-def apply_3(f: Int => String): Bool = f(3) == "many"
-
-scala> apply_3(mk_string)
-res6 = true
-\end{lstlisting}
-
-You might ask: Apart from silly functions like above, what is
-the point of having functions as input arguments to other
-functions? In Java there is indeed no need of this kind of
-feature: at least in the past it did not allow such
-constructions. I think, the point of Java 8 is to lift this
-restriction. But in all functional programming languages,
-including Scala, it is really essential to allow functions as
-input argument. Above you already 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 \code{foreach}
-with either of them and thus changing how, for example, five
-numbers are printed.
-
-
-\begin{lstlisting}[numbers=none]
-scala> (1 to 5).toList.foreach(print)
-12345
-scala> (1 to 5).toList.foreach(println)
-1
-2
-3
-4
-5
-\end{lstlisting}
-
-
-\noindent This is actually one of the main design principles
-in functional programming. You have generic functions like
-\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
-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 \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 \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 \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 \code{print}.
-
-
-\subsection*{Cool Stuff}
-
-The first wow-moment I had with Scala was when I came across
-the following code-snippet for reading a web-page. 
-
-
-\begin{lstlisting}[ numbers=none]
-import io.Source
-val url = """http://www.inf.kcl.ac.uk/staff/urbanc/"""
-Source.fromURL(url)("ISO-8859-1").take(10000).mkString
-\end{lstlisting}
-
-
-\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 it 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
-\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{lstlisting}[numbers=none]
-SEQ(CHAR('f'), SEQ(CHAR('o'), CHAR('r')))
-\end{lstlisting}
-
-\noindent This gets quickly unreadable when the strings and
-regular expressions get more complicated. In other functional
-programming languages, you can explicitly write a conversion
-function that takes a string, say \dq{\pcode{for}}, and
-generates the regular expression above. But then your code is
-littered with such conversion functions.
-
-In Scala you can do better by ``hiding'' the conversion
-functions. The keyword for doing this is \code{implicit} and
-it needs a built-in library called 
-
-\begin{lstlisting}[numbers=none]
-scala.language.implicitConversions
-\end{lstlisting}
-
-\noindent
-Consider the code
-
-
-\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}
-
-
-\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
-\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 \code{string2rexp}-function. I can now
-write for example
-
-\begin{lstlisting}[numbers=none]
-scala> ALT("ab", "ac")
-res9 = ALT(SEQ(CHAR(a),CHAR(b)),SEQ(CHAR(a),CHAR(c)))
-\end{lstlisting}
-
-\noindent Recall that \code{ALT} expects two regular
-expressions as arguments, but I only supply two strings. The
-implicit conversion function will transform the string into a
-regular expression.
-
-Using implicit definitions, Scala allows me to introduce
-some further syntactic sugar for regular expressions:
-
-
-\begin{lstlisting}[ numbers=none]
-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}
-
- 
-\noindent This might seem a bit overly complicated, but its effect is
-that I can now write regular expressions such as $ab + ac$ 
-simply as
-
-
-\begin{lstlisting}[numbers=none]
-scala> "ab" | "ac"
-res10 = ALT(SEQ(CHAR(a),CHAR(b)),SEQ(CHAR(a),CHAR(c)))
-\end{lstlisting}
-
- 
-\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 varying 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{lstlisting}[numbers=none]
-ALT(..., ALT(..., ALT(..., ...)))
-\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 \code{*} to the type of an argument.
-Consider the code
-
-
-\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}
-
-
-\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{lstlisting}[numbers=none]
-ALTS("for", "def", "yield", "implicit", "if", "match", "case")
-\end{lstlisting}
-
-
-\noindent Again I leave it to you to find out how much this
-simplifies the regular expression in comparison with if I had
-to write this by hand using only the ``plain'' regular
-expressions from the inductive datatype.
-
-\subsection*{More Info}
-
-There is much more to Scala than I can possibly describe in
-this document. Fortunately there are a number of free books
-about Scala and of course lots of help online. For example
-
-\begin{itemize}
-\item \url{http://www.scala-lang.org/docu/files/ScalaByExample.pdf}
-\item \url{http://www.scala-lang.org/docu/files/ScalaTutorial.pdf}
-\item \url{https://www.youtube.com/user/ShadowofCatron}
-\item \url{http://docs.scala-lang.org/tutorials}
-\item \url{https://www.scala-exercises.org}
-\end{itemize}
-
-\noindent There is also a course at Coursera on Functional
-Programming Principles in Scala by Martin Odersky, the main
-developer of the Scala language. And a document that explains
-Scala for Java programmers
-
-\begin{itemize}
-\item \small\url{http://docs.scala-lang.org/tutorials/scala-for-java-programmers.html}
-\end{itemize}
-
-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
-\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{lstlisting}[numbers=none]
-scala>  List (1, 2, 3) contains "your mom"
-res1: Boolean = false
-\end{lstlisting}
-
-\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. One can work around this
-particular limitation, but why does one have to?
-More such `puzzles' can be found at
-
-\begin{center}
-  \url{http://scalapuzzlers.com} and
-  \url{http://latkin.org/blog/2017/05/02/when-the-scala-compiler-doesnt-help/}
-\end{center}
-
-Even if Scala has been a success in several high-profile
-companies, there is also a company (Yammer) that first used
-Scala in their production code, but then moved away from it.
-Allegedly they did not like the steep learning curve of Scala
-and also that new versions of Scala often introduced
-incompatibilities in old code. In the past two months
-there have also been two forks of the Scala compiler.
-It needs to be seen what the future brings for Scala.
-
-So all in all, Scala might not be a great teaching language,
-but I hope this is mitigated by the fact that I never require
-you to write any Scala code. You only need to be able to read
-it. In the coursework you can use any programming language you
-like. If you want to use Scala for this, then be my guest; if
-you do not want, stick with the language you are most familiar
-with.
-
-
-
-\end{document}
-
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