afl-material: comparison handouts/scala-ho.tex-old

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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}
+%%% Local Variables:
+%%% mode: latex
+%%% TeX-master: t
+%%% End:

changeset 871	358a72d7bf71
parent 488	057b4603b940