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%cheat sheet
%http://worldline.github.io/scala-cheatsheet/

% case class, apply, unapply
% see https://medium.com/@thejasbabu/scala-pattern-matching-9c9e73ba9a8a

% the art of programming
% https://www.youtube.com/watch?v=QdVFvsCWXrA

% functional programming in Scala
%https://www.amazon.com/gp/product/1449311032/ref=as_li_ss_tl?ie=UTF8&tag=aleottshompag-20&linkCode=as2&camp=1789&creative=390957&creativeASIN=1449311032

% functional programming in C
%https://www.amazon.com/gp/product/0201419505/ref=as_li_ss_tl?ie=UTF8&camp=1789&creative=390957&creativeASIN=0201419505&linkCode=as2&tag=aleottshompag-20

%speeding through haskell
%https://openlibra.com/en/book/download/speeding-through-haskell

% fp books --- ocaml
% http://courses.cms.caltech.edu/cs134/cs134b/book.pdf
% http://alexott.net/en/fp/books/

%John Hughes’ simple words:
%A combinator is a function which builds program fragments
%from program fragments.


%explain graph colouring program (examples from)
%https://www.metalevel.at/prolog/optimization

% nice example for map and reduce using Harry potter characters
% https://www.matthewgerstman.com/map-filter-reduce/

% interesting talk about differences in Java and Scala
% Goto'19 conference ; about differences in type-system
% https://www.youtube.com/watch?v=e6n-Ci8V2CM

% Timing
%
% xs.map(x => (x, xs.count(_==x)))
%
% vs  xs.groupBy(identity)
%
% first is quadratic, while second is linear.

% contrast map with a for loop in imperative languages
%
% Let’s use a simple example of calculating sales tax on an array of
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%
%       const prices = [19.99, 4.95, 25, 3.50];
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%
%       for(let i=0; i < prices.length; i++) {
%          new_prices.push(prices[i] * 1.06);
%       }
%
% We can achieve the same results using .map():
%
% const prices = [19.99, 4.95, 25, 3.50]; 
% let new_prices = prices.map(price => price * 1.06);
%
% The syntax above is condensed so let’s walk through it a bit. The
% .map() method takes a callback, which can be thought of as a function.
% That’s what is between the parentheses. The variable price is the name
% that will be used to identify each value. Since there’s only one
% input, we can omit the usual parentheses around the parameters.

% potentially a worked example? Tetris in scala.js
%  
% https://medium.com/@michael.karen/learning-modern-javascript-with-tetris-92d532bcd057
%
% Scala videos
%    https://www.youtube.com/user/DrMarkCLewis


%% https://alvinalexander.com/downloads/HelloScala-FreePreview.pdf
%% 
%% Section 10 about strings; interpolations and multiline strings

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\begin{document}
\fnote{\copyright{} Christian Urban, King's College London, 2017, 2018, 2019, 2020}

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\section*{A Crash-Course in Scala}

\mbox{}\hfill\textit{``Scala --- \underline{S}lowly \underline{c}ompiled 
\underline{a}cademic \underline{la}nguage''}\smallskip\\
\mbox{}\hfill\textit{ --- a joke(?) found on Twitter}\bigskip

\subsection*{Introduction}

\noindent
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 or so years. 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. Because of this it has also access to
the myriads of Java libraries. Unlike Java, however, Scala often allows
programmers to write very concise and elegant code.  Some therefore say
``Scala is the better Java''.\footnote{from
\url{https://www.slideshare.net/maximnovak/joy-of-scala}} 

A number of companies---the Guardian, Twitter, Coursera, FourSquare,
Netflix, LinkedIn, ITV to name a few---either use Scala exclusively in
production code, or at least to some substantial degree. Scala seems
also useful in job-interviews (especially in data science) according to
this anecdotal report

\begin{quote}
\url{http://techcrunch.com/2016/06/14/scala-is-the-new-golden-child}
\end{quote}

\noindent
The official Scala compiler can be downloaded from

\begin{quote}
\url{http://www.scala-lang.org}\medskip
\end{quote}

\noindent
If you are interested, there are also experimental backends of Scala
for producing code under Android (\url{http://scala-android.org}); for
generating JavaScript code (\url{https://www.scala-js.org}); and there
is work under way to have a native Scala compiler generating X86-code
(\url{http://www.scala-native.org}). Though be warned these backends
are still rather beta or even alpha.

\subsection*{VS Code and Scala}

I found a convenient IDE for writing Scala programs is Microsoft's
\textit{Visual Studio Code} (VS Code) which runs under MacOSX, Linux and
obviously Windows.\footnote{\ldots{}unlike \emph{Microsoft Visual Studio}---note
the minuscule difference in the name---which is a heavy-duty,
Windows-only IDE\ldots{}jeez, with all their money could they not have come
up with a completely different name for a complete different project?
For the pedantic, Microsoft Visual Studio is an IDE, whereas Visual
Studio Code is considered to be a \emph{source code editor}. Anybody knows what the
difference is?} It can be downloaded for free from

\begin{quote}
\url{https://code.visualstudio.com}
\end{quote}

\noindent
and should already come pre-installed in the Department (together with
the Scala compiler). Being a project that just started in 2015, VS Code is
relatively new and thus far from perfect. However it includes a
\textit{Marketplace} from which a multitude of extensions can be
downloaded that make editing and running Scala code a little easier (see
Figure~\ref{vscode} for my setup).

\begin{figure}[t]
\begin{boxedminipage}{\textwidth}  
\begin{center}  
\includegraphics[scale=0.15]{../pics/vscode.png}\\[-10mm]\mbox{}
\end{center}
\caption{My installation of VS Code includes the following
  packages from Marketplace: \textbf{Scala Syntax (official)} 0.3.4,
  \textbf{Code Runner} 0.9.13, \textbf{Code Spell Checker} 1.7.17,
  \textbf{Rewrap} 1.9.1 and \textbf{Subtle Match
  Brackets} 3.0.0. I have also bound the keys \keys{Ctrl} \keys{Ret} to the
  action ``Run-Selected-Text-In-Active-Terminal'' in order to quickly
  evaluate small code snippets in the Scala REPL. I use the internal
  terminal to run Scala 2.13.1.\label{vscode}}
\end{boxedminipage}
\end{figure}  

What I like most about VS Code is that it provides easy access to the
Scala REPL. But if you prefer another editor for coding, it is also
painless to work with Scala completely on the command line (as you might
have done with \texttt{g++} in the earlier part of PEP). For the
lazybones among us, there are even online editors and environments for
developing and running Scala programs: \textit{ScalaFiddle}
and \textit{Scastie} are two of them. They require zero setup 
(assuming you have a browser handy). You can access them at 
 
\begin{quote}
  \url{https://scalafiddle.io}\\
  \url{https://scastie.scala-lang.org}\medskip
\end{quote}
  
\noindent
But you should be careful if you use them for your coursework: they
are meant to play around, not really for serious work. 

As one might expect, Scala can be used with the heavy-duty IDEs Eclipse and IntelliJ.
A ready-made Scala bundle for Eclipse is available from

\begin{quote}
\url{http://scala-ide.org/download/sdk.html}
\end{quote}

\noindent
Also IntelliJ includes plugins for Scala. \underline{\textbf{BUT}}, 
I do \textbf{not} recommend the usage of either Eclipse or IntelliJ for PEP: these IDEs
seem to make your life harder, rather than easier, for the small
programs that we will write in this module. They are really meant to be used
when you have a million-lines codebase than with our small
``toy-programs''\ldots{}for example why on earth am I required to create a
completely new project with several subdirectories when I just want to
try out 20-lines of Scala code? Your mileage may vary though.~\texttt{;o)}

\subsection*{Why Functional Programming?}

Before we go on, let me explain a bit more why we want to inflict upon
you another programming language. You hopefully have mastered Java and
C++\ldots{}the world should be your oyster, no? Well, matters are not as
simple as one might wish. We do require Scala in PEP, but actually we do
not religiously care whether you learn Scala---after all it is just a
programming language (albeit a nifty one IMHO). What we do care about is
that you learn about \textit{functional programming}. Scala is just the
vehicle for that. Still, you need to learn Scala well enough to get good
marks in PEP, but functional programming could perhaps equally be taught
with Haskell, F\#, SML, Ocaml, Kotlin, Clojure, Scheme, Elm and many
other functional programming languages. 

%Your friendly lecturer just
%happens to like Scala and the Department agreed that it is a good idea
%to inflict Scala upon you.

Very likely writing programs in a functional programming language is
quite different from what you are  used to in your study so far. It
might even be totally alien to you. The reason is that functional
programming seems to go against the core principles of
\textit{imperative programming} (which is what you do in Java and C/C++
for example). The main idea of imperative programming  is that you have
some form of \emph{state} in your program and you continuously change
this state by issuing some commands---for example for updating a field
in an array or for adding one to a variable and so on. The classic
example for this style of programming is a \texttt{for}-loop in C/C++.
Consider the snippet:

\begin{lstlisting}[language=C,numbers=none]
for (int i = 10; i < 20; i++) { 
      //...do something with i...
}
\end{lstlisting}

\noindent Here the integer variable \texttt{i} embodies the state, which
is first set to \texttt{10} and then increased by one in each
loop-iteration until it reaches \texttt{20} at which point the loop
exits. When this code is compiled and actually runs, there will be some
dedicated space reserved for \texttt{i} in memory. This space of
typically 32 bits contains \texttt{i}'s current value\ldots\texttt{10}
at the beginning, and then the content will be overwritten with 
new content in every iteration. The main point here is that this kind of
updating, or overwriting, of memory is 25.806\ldots or \textbf{THE ROOT OF
ALL EVIL}!!

\begin{center}
\includegraphics[scale=0.25]{../pics/root-of-all-evil.png}
\end{center}  


\noindent
\ldots{}Well, it is perfectly benign if you have a sequential program
that gets run instruction by instruction...nicely one after another.
This kind of running code uses a single core of your CPU and goes as
fast as your CPU frequency, also called clock-speed, allows. The problem
is that this clock-speed has not much increased over the past decade and
no dramatic increases are predicted for any time soon. So you are a bit
stuck. This is unlike previous generations of developers who could rely
upon the fact that approximately every 2 years their code would run
twice as fast  because the clock-speed of their CPUs got twice as fast.

Unfortunately this does not happen any more nowadays. To get you out of
this dreadful situation, CPU producers pile more and more cores into
CPUs in order to make them more powerful and potentially make software
faster. The task for you as developer is to take somehow advantage of
these cores by running as much of your code as possible in parallel on
as many cores you have available (typically 4 or more in modern laptops
and sometimes much more on high-end machines). In this situation
\textit{mutable} variables like \texttt{i} in the C-code above are evil,
or at least a major nuisance: Because if you want to distribute some of
the loop-iterations over several cores that are currently idle in your
system, you need to be extremely careful about who can read and
overwrite the variable \texttt{i}.\footnote{If you are of the mistaken
belief that nothing nasty can happen to \texttt{i} inside the
\texttt{for}-loop, then you need to go back over the C++ material.}
Especially the writing operation is critical because you do not want
that conflicting writes mess about with \texttt{i}. Take my word: an
untold amount of misery has arisen from this problem. The catch is that
if you try to solve this problem in C/C++ or Java, and be as defensive
as possible about reads and writes to \texttt{i}, then you need to
synchronise access to it. The result is that very often your program
waits more than it runs, thereby defeating the point of trying to run
the program in parallel in the first place. If you are less defensive,
then usually all hell breaks loose by seemingly obtaining random
results. And forget the idea of being able to debug such code.

The central idea of functional programming is to eliminate any state
from programs---or at least from the ``interesting bits'' of the
programs. Because then it is easy to parallelise the resulting
programs: if you do not have any state, then once created, all memory
content stays unchanged and reads to such memory are absolutely safe
without the need of any synchronisation. An example is given in
Figure~\ref{mand} where in the absence of the annoying state, Scala
makes it very easy to calculate the Mandelbrot set on as many cores of
your CPU as possible. Why is it so easy in this example? Because each
pixel in the Mandelbrot set can be calculated independently and the
calculation does not need to update any variable. It is so easy in
fact that going from the sequential version of the Mandelbrot program
to the parallel version can be achieved by adding just eight
characters---in two places you have to add \texttt{.par}. Try the same
in C/C++ or Java!

\begin{figure}[p]
\begin{boxedminipage}{\textwidth}

A Scala program for generating pretty pictures of the Mandelbrot set.\smallskip\\ 
(See \url{https://en.wikipedia.org/wiki/Mandelbrot_set} or\\
\phantom{(See }\url{https://www.youtube.com/watch?v=aSg2Db3jF_4}):
\begin{center}    
\begin{tabular}{c}  
\includegraphics[scale=0.11]{../pics/mand1.png}\\[-8mm]\mbox{}
\end{tabular}
\end{center}

\begin{center}
\begin{tabular}{@{}p{0.45\textwidth}|p{0.45\textwidth}@{}}
  \bf sequential version: & \bf parallel version on 4 cores:\smallskip\\

  {\hfill\includegraphics[scale=0.11]{../pics/mand4.png}\hfill} &
  {\hfill\includegraphics[scale=0.11]{../pics/mand3.png}\hfill} \\

{\footnotesize\begin{lstlisting}[xleftmargin=-1mm]
for (y <- (0 until H)) {
  for (x <- (0 until W)) {
    
    val c = start + 
      (x * d_x + y * d_y * i)
    val iters = iterations(c, max) 
    val colour = 
      if (iters == max) black 
      else colours(iters % 16)

    pixel(x, y, colour)
  }
  viewer.updateUI()
}   
\end{lstlisting}}   
& 
{\footnotesize\begin{lstlisting}[xleftmargin=0mm]
for (y <- (0 until H)/*@\keys{\texttt{.par}}@*/) {
  for (x <- (0 until W)/*@\keys{\texttt{.par}}@*/) {
      
    val c = start + 
      (x * d_x + y * d_y * i)
    val iters = iterations(c, max) 
    val colour = 
      if (iters == max) black 
      else colours(iters % 16)
  
    pixel(x, y, colour)
  }
  viewer.updateUI()
}   
\end{lstlisting}}\\[-2mm]

\centering\includegraphics[scale=0.5]{../pics/cpu2.png} &
\centering\includegraphics[scale=0.5]{../pics/cpu1.png}
\end{tabular}
\end{center}
\caption{The code of the ``main'' loops in my version of the mandelbrot program.
The parallel version differs only in \texttt{.par} being added to the
``ranges'' of the x and y coordinates. As can be seen from the CPU loads, in
the sequential version there is a lower peak for an extended period,
while in the parallel version there is a short sharp burst for
essentially the same workload\ldots{}meaning you get more work done 
in a shorter amount of time. This easy \emph{parallelisation} 
only works reliably with an immutable program.
\label{mand}} 
\end{boxedminipage}
\end{figure}  

But remember this easy parallelisation of code requires that we have no
state in our programs\ldots{}that is no counters like \texttt{i} in
\texttt{for}-loops. You might then ask, how do I write loops without
such counters? Well, teaching you that this is possible is one of the
main points of the Scala-part in PEP. I can assure you it is possible,
but you have to get your head around it. Once you have mastered this, it
will be fun to have no state in your programs (a side product is that it
much easier to debug state-less code and also more often than not easier
to understand). So have fun with Scala!\footnote{If you are still not
convinced about the function programming ``thing'', there are a few more
arguments: a lot of research in programming languages happens to take
place in functional programming languages. This has resulted in
ultra-useful features such as pattern-matching, strong type-systems,
laziness, implicits, algebraic datatypes  to name a few. Imperative
languages seem to often lag behind in adopting them: I know, for
example, that Java will at some point in the future support
pattern-matching, which has been used for example in SML for at least
40(!) years. See
\url{http://cr.openjdk.java.net/~briangoetz/amber/pattern-match.html}.
Automatic garbage collection was included in Java in 1995; the
functional language LISP had this already in 1958. Generics were added
to Java 5 in 2004; the functional language SML had it since 1990.
Higher-order functions were added to C\# in 2007, to Java 8 in
2014; again LISP had them since 1958. Also Rust, a C-like programming
language that has been developed since 2010 and is gaining quite some
interest, borrows many ideas from functional programming from
yesteryear.}\medskip

\noindent
If you need any after-work distractions, you might have fun reading this
about FP (functional programming):

\begin{quote}
\url{https://medium.com/better-programming/fp-toy-7f52ea0a947e}
\end{quote}

\subsection*{The Very Basics}

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 a compiler. This helps a lot with interactively developing
programs. It is my preferred way of writing small Scala
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 2.13.1 (Java HotSpot(TM) 64-Bit Server VM, Java 9).
Type in expressions for evaluation. Or try :help.

scala>
\end{lstlisting}%$



\noindent The precise response may vary depending
on 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 The answer means that he result of the addition is of type
\code{Int} and the actual result is 5; \code{res0} is a name that
Scala gives automatically to the result. You can reuse this name later
on, for example

\begin{lstlisting}[numbers=none]
scala> res0 + 4
res1: Int = 9
\end{lstlisting}

\noindent
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{resX} 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. It is just not printed by Scala. 

You can try more examples with the Scala REPL, but feel free to
first guess what the result is (not all answers by Scala are obvious):

\begin{lstlisting}[numbers=none]
scala> 2 + 2
scala> 1 / 2
scala> 1.0 / 2
scala> 1 / 2.0
scala> 1 / 0
scala> 1.0 / 0.0
scala> true == false
scala> true && false
scala> 1 > 1.0
scala> "12345".length
scala> List(1,2,1).size
scala> Set(1,2,1).size
scala> List(1) == List(1)
scala> Array(1) == Array(1)
scala> Array(1).sameElements(Array(1))
\end{lstlisting}

\noindent
Also observe carefully what Scala responds in the following 
three instances involving the constant \lstinline!1!---can 
you explain the differences?


\begin{lstlisting}[numbers=none]
scala> 1
scala> 1L
scala> 1F
\end{lstlisting}\smallskip



\noindent
Please take the Scala REPL seriously: If you want to take advantage of my
reference implementation for the assignments, you will need to be
able to ``play around'' with it!

\subsection*{Standalone Scala Apps}

If you want to write a standalone app in Scala, you can
implement an object that is an instance of \code{App}. For example
write

\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 (\texttt{scalac}) and start the runtime
environment (\texttt{scala}):

\begin{lstlisting}[language={},numbers=none,basicstyle=\ttfamily\small]
$ scalac hello-world.scala
$ scala Hello
hello world
\end{lstlisting}

\noindent
Like Java, 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*{Values}

In the lectures I will try to avoid as much as possible the term
\emph{variables} familiar from other programming languages. The reason
is that Scala has \emph{values}, which can be seen as abbreviations of
larger expressions. The keyword for defining values is \code{val}.
For example

\begin{lstlisting}[numbers=none]
scala> val x = 42
x: Int = 42

scala> val y = 3 + 4
y: Int = 7

scala> val z = x / y
z: Int = 6
\end{lstlisting}

\noindent
As can be seen, we first define \code{x} and {y} with admittedly some silly
expressions, and then reuse these values in the definition of \code{z}.
All easy, right? Why the kerfuffle about values? Well, values are
\emph{immutable}. You cannot change their value after you defined them.
If you try to reassign \code{z} above, Scala will yell at you:

\begin{lstlisting}[numbers=none]
scala> z = 9
error: reassignment to val
       z = 9
         ^
\end{lstlisting}

\noindent
So it would be a bit absurd to call values as variables...you cannot
change them; they cannot vary. You might think you can reassign them like

\begin{lstlisting}[numbers=none]
scala> val x = 42
scala> val z = x / 7
scala> val x = 70
scala> println(z) 
\end{lstlisting}

\noindent but try to guess what Scala will print out 
for \code{z}?  Will it be \code{6} or \code{10}? A final word about
values: Try to stick to the convention that names of values should be
lower case, like \code{x}, \code{y}, \code{foo41} and so on. Upper-case
names you should reserve for what is called \emph{constructors}. And 
forgive me when I call values as variables\ldots{}it is just something that
has been in imprinted into my developer-DNA during my early days and
is difficult to get rid of.~\texttt{;o)}  


\subsection*{Function Definitions}

We do functional programming! So defining functions will be our main occupation.
As an example, a function named \code{f} taking a single argument of type 
\code{Int} can be defined in Scala as follows:

\begin{lstlisting}[numbers=none]
def f(x: Int) : String = ...EXPR...
\end{lstlisting} 

\noindent
This function returns the value resulting from evaluating the expression
\code{EXPR} (whatever is substituted for this). Since we declared
\code{String}, the result of this function will be of type
\code{String}. It is a good habit to always include this information
about the return type, while it is only strictly necessary to give this
type in recursive functions. Simple examples of Scala functions are:

\begin{lstlisting}[numbers=none]
def incr(x: Int) : Int = x + 1
def double(x: Int) : Int = x + x
def square(x: Int) : Int = x * x
\end{lstlisting}

\noindent
The general scheme for a function is

\begin{lstlisting}[numbers=none]
def fname(arg1: ty1, arg2: ty2,..., argn: tyn): rty = {
  ...BODY...
}
\end{lstlisting}

\noindent
where each argument, \texttt{arg1}, \texttt{arg2} and so on, requires 
its type and the result type of the
function, \code{rty}, should also be given. If the body of the function is
more complex, then it can be enclosed in braces, like above. If it it
is just a simple expression, like \code{x + 1}, you can omit the
braces. Very often functions are recursive (that is call themselves),
like the venerable factorial function:

\begin{lstlisting}[numbers=none]
def fact(n: Int) : Int = 
  if (n == 0) 1 else n * fact(n - 1)
\end{lstlisting}

\noindent
We could also have written this with braces as

\begin{lstlisting}[numbers=none]
def fact(n: Int) : Int = {
  if (n == 0) 1 
  else n * fact(n - 1)
}    
\end{lstlisting}

\noindent
but this seems a bit overkill for a small function like \code{fact}.
Note that Scala does not have a \code{then}-keyword in an
\code{if}-statement. Also important is that there should be always an
\code{else}-branch. Never write an \code{if} without an \code{else},
unless you know what you are doing! While \code{def} is the main
mechanism for defining functions, there are a few other ways for doing
this. We will see some of them in the next sections.

Before we go on, let me explain one tricky point in function
definitions, especially in larger definitions. What does a Scala
function return as result? Scala has a \code{return} keyword, but it is
used for something different than in Java (and C/C++). Therefore please
make sure no \code{return} slips into your Scala code.

So in the absence of \code{return}, what value does a Scala function
actually produce? A rule-of-thumb is whatever is in the last line of the
function is the value that will be returned. Consider the following
example:\footnote{We could have written this function in just one line,
but for the sake of argument lets keep the two intermediate values.}

\begin{lstlisting}[numbers=none]
def average(xs: List[Int]) : Int = {
  val s = xs.sum
  val n = xs.length
  s / n
}    
\end{lstlisting}

\noindent In this example the expression \code{s / n} is in the last
line of the function---so this will be the result the function
calculates. The two lines before just calculate intermediate values.
This principle of the ``last-line'' comes in handy when you need to
print out values, for example, for debugging purposes. Suppose you want
rewrite the function as

\begin{lstlisting}[numbers=none]
def average(xs: List[Int]) : Int = {
  val s = xs.sum
  val n = xs.length
  val h = xs.head
  println(s"Input $xs with first element $h")
  s / n
}    
\end{lstlisting}

\noindent
Here the function still only returns the expression in the last line.
The \code{println} before just prints out some information about the
input of this function, but does not contribute to the result of the
function. Similarly, the value \code{h} is used in the \code{println}
but does not contribute to what integer is returned. 

A caveat is that the idea with the ``last line'' is only a rough
rule-of-thumb. A better rule might be: the last expression that is
evaluated in the function. Consider the following version of
\code{average}:

\begin{lstlisting}[numbers=none]
def average(xs: List[Int]) : Int = {
  if (xs.length == 0) 0
  else xs.sum / xs.length
}    
\end{lstlisting}

\noindent
What does this function return? Well there are two possibilities: either
the result of \code{xs.sum / xs.length} in the last line provided the
list \code{xs} is nonempty, \textbf{or} if the list is empty, then it
will return \code{0} from the \code{if}-branch (which is technically not
the last line, but the last expression evaluated by the function in the
empty-case).

Summing up, do not use \code{return} in your Scala code! A function
returns what is evaluated by the function as the last expression. There
is always only one such last expression. Previous expressions might
calculate intermediate values, but they are not returned. If your
function is supposed to return multiple things, then one way in Scala is
to use tuples. For example returning the minimum, average and maximum
can be achieved by

\begin{lstlisting}[numbers=none]
def avr_minmax(xs: List[Int]) : (Int, Int, Int) = {
  if (xs.length == 0) (0, 0, 0)
  else (xs.min, xs.sum / xs.length, xs.max)
}    
\end{lstlisting}

\noindent
which still satisfies the rule-of-thumb.


\subsection*{Loops, or Better the Absence Thereof}

Coming from Java or C/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 corresponding 
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 (in this example the function is $n \rightarrow n * n$) and
a list over which this function should work. Pictorially you can think
of the idea behind maps as follows:

\begin{center}
\begin{tikzpicture}
                      
  \node (A0) at (1.2,0) {\texttt{List(}};
  \node (A1) at (2.0,0) {\texttt{1\makebox[0mm]{ ,}}};
  \node (A2) at (2.9,0) {\texttt{2\makebox[0mm]{ ,}}};
  \node (A3) at (3.8,0) {\texttt{3\makebox[0mm]{ ,}}};
  \node (A4) at (4.7,0) {\texttt{4\makebox[0mm]{ ,}}};
  \node (A5) at (5.6,0) {\texttt{5\makebox[0mm]{ ,}}};
  \node (A6) at (6.5,0) {\texttt{6\makebox[0mm]{ ,}}};
  \node (A7) at (7.4,0) {\texttt{7\makebox[0mm]{ ,}}};
  \node (A8) at (8.3,0) {\texttt{8)}};

  \node (B0) at (1.2,-3) {\texttt{List(}};
  \node (B1) at (2.0,-3) {\texttt{1\makebox[0mm]{ ,}}};
  \node (B2) at (3.0,-3) {\texttt{4\makebox[0mm]{ ,}}};
  \node (B3) at (4.1,-3) {\texttt{9\makebox[0mm]{ ,}}};
  \node (B4) at (5.2,-3) {\texttt{16\makebox[0mm]{ ,}}};
  \node (B5) at (6.3,-3) {\texttt{25\makebox[0mm]{ ,}}};
  \node (B6) at (7.4,-3) {\texttt{36\makebox[0mm]{ ,}}};
  \node (B7) at (8.4,-3) {\texttt{49\makebox[0mm]{ ,}}};
  \node (B8) at (9.4,-3) {\texttt{64\makebox[0mm]{ )}}};

  \draw [->,line width=1mm] (A1.south) -- (B1.north);
  \draw [->,line width=1mm] (A2.south) -- (B2.north);
  \draw [->,line width=1mm] (A3.south) -- (B3.north);
  \draw [->,line width=1mm] (A4.south) -- (B4.north);
  \draw [->,line width=1mm] (A5.south) -- (B5.north);
  \draw [->,line width=1mm] (A6.south) -- (B6.north);
  \draw [->,line width=1mm] (A7.south) -- (B7.north);
  \draw [->,line width=1mm] (A8.south) -- (B8.north);

  \node [red] (Q0) at (-0.3,-0.3) {\large\texttt{n}}; 
  \node (Q1) at (-0.3,-0.4) {};
  \node (Q2) at (-0.3,-2.5) {};
  \node [red] (Q3) at (-0.3,-2.65) {\large\texttt{n\,*\,n}};
  \draw [->,red,line width=1mm] (Q1.south) -- (Q2.north);

  \node [red] at (-1.3,-1.5) {\huge{}\it\textbf{map}};
 \end{tikzpicture}
\end{center}

\noindent
On top is the ``input'' list we want to transform; on the left is the
``map'' function for how to transform each element in the input list
(the square function in this case); at the bottom is the result list of
the map. This means that a map generates a \emph{new} list, unlike a
for-loop in Java or C/C++ which would most likely just update the
existing list/array.

Now there are two ways for expressing such maps in Scala. The first way is
called a \emph{for-comprehension}. The keywords are \code{for} and
\code{yield}. Squaring the numbers from 1 to 8 with a for-comprehension
would look 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  This for-comprehension states that from the list of numbers
we draw some elements. We use the name \code{n} to range over these
elements (whereby the name is arbitrary; we could use something more
descriptive if we wanted to). Using \code{n} we compute the result of
\code{n * n} after the \code{yield}. This way of writing a map resembles
a bit the for-loops from imperative languages, even though the ideas
behind for-loops and for-comprehensions are quite different. Also, this
is a simple example---what comes after \code{yield} can be a complex
expression enclosed in \texttt{\{...\}}. A more complicated example
might be

\begin{lstlisting}[numbers=none]
scala> for (n <- (1 to 8).toList) yield {
         val i = n + 1
         val j = n - 1
         i * j + 1
       }
res3: List[Int] = List(1, 4, 9, 16, 25, 36, 49, 64)
\end{lstlisting}

As you can see in for-comprehensions above, 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 the function \code{map} 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 by the map).  It might not be obvious, but
the for-comprehensions above are just syntactic sugar: when compiling such
code, 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 remainders 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 remainders
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\footnote{This returns actually \code{HashSet(2, 1, 3)},
but this is just an implementation detail of how sets are implemented in
Scala.} 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 
In this example the for-comprehension ranges over two lists, and
produces a list of pairs as output. 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 this for-comprehension filters out
all pairs where the sum is not even (therefore \code{(1, 2)}, \code{(2,
1)} and \code{(3, 2)} are not in the result because their sum is odd). 

To summarise, maps (or for-comprehensions) transform one collection into
another. For example a list of \code{Int}s into a list of squares, and
so on. There is no need for for-loops in Scala. But please do not be
tempted to write anything like

\begin{lstlisting}[numbers=none]
scala> val cs = ('a' to 'h').toList
scala> for (n <- (0 until cs.length).toList) 
          yield cs(n).capitalize
res8: List[Char] = List(A, B, C, D, E, F, G, H)
\end{lstlisting}

\noindent
This is accepted Scala-code, but utterly bad style (it is more like
Java). It can be written much clearer as:

\begin{lstlisting}[numbers=none]
scala> val cs = ('a' to 'h').toList
scala> for (c <- cs) yield c.capitalize
res9: List[Char] = List(A, B, C, D, E, F, G, H)
\end{lstlisting}

\subsection*{Results and Side-Effects}

While hopefully all this about maps 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 remainders modulo 3). What happens if we
just want to print out a list of integers? In these cases the
for-comprehensions need 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 \emph{side-effect}\ldots it
prints something out on the screen. 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
  println(s"$n * $n = $square") 
}
1 * 1 = 1
2 * 2 = 4
3 * 3 = 9
4 * 4 = 16
5 * 5 = 25
\end{lstlisting}%$

\noindent In this code I use a value assignment (\code{val
square = ...} ) 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} 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 
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*{Aggregates}

There is one more usage of for-loops in Java, C/C++ and the like:
sometimes you want to \emph{aggregate} something about a list, for
example summing up all its elements. In this case you cannot use maps,
because maps \emph{transform} one data collection into another data
collection. They cannot be used to generate a single integer
representing an aggregate. So how is this kind of aggregation done in
Scala? Let us suppose you want to sum up all elements from a list. You
might be tempted to write something like

\begin{lstlisting}[numbers=none]
var cnt = 0
for (n <- (1 to 8).toList) {
  cnt += n
}
print(cnt)
\end{lstlisting}

\noindent
and indeed this is accepted Scala code and produces the expected result,
namely \code{36}, \textbf{BUT} this is imperative style and not
permitted in PEP. If you submit this kind of code, you get 0 marks. The
code uses a \code{var} and therefore violates the immutability property
I ask for in your code. Sorry!

So how to do that same thing without using a \code{var}? Well there are
several ways. One way is to define the following recursive
\code{sum}-function:

\begin{lstlisting}[numbers=none]
def sum(xs: List[Int]) : Int = 
  if (xs.isEmpty) 0 else xs.head + sum(xs.tail)
\end{lstlisting}  

\noindent
You can then call \code{sum((1 to 8).toList)} and obtain the same result
without a mutable variable and without a for-loop. Obviously for simple things like
sum, you could have written \code{xs.sum} in the first place. But not
all aggregate functions are pre-defined and often you have to write your
own recursive function for this.

%\subsection*{Always Produce a Result! No Exceptions!}
%
%Function should always produce a value. Exception is not thrown.
%Whenever there is a possibility of non-value result (exception, void,
%undefined, null, etc.), it should be incorporated in the result type.
%Such types include but not limited to
%
%Option[T]

%TBD


\subsection*{Higher-Order Functions}

Functions obviously play a central role in functional programming. Two simple
examples are

\begin{lstlisting}[numbers=none]
def even(x: Int) : Boolean = x % 2 == 0
def odd(x: Int) : Boolean = x % 2 == 1
\end{lstlisting} 

\noindent
More interestingly, the concept of functions is really pushed to the
limit in functional programming. Functions can take other functions as
arguments and can return a function as a result. This is actually
quite important for making code generic. Assume a list of 10 elements:

\begin{lstlisting}[numbers=none]
val lst = (1 to 10).toList  
\end{lstlisting} 

\noindent 
Say, we want to filter out all even numbers. For this we can use 

\begin{lstlisting}[numbers=none]
scala> lst.filter(even)
List(2, 4, 6, 8, 10)
\end{lstlisting} 

\noindent
where \code{filter} expects a function as argument specifying which
elements of the list should be kept and which should be left out. By
allowing \code{filter} to take a function as argument, we can also
easily filter out odd numbers as well.

\begin{lstlisting}[numbers=none]
scala> lst.filter(odd)
List(1, 3, 5, 7, 9)
\end{lstlisting} 

\noindent
Such function arguments are quite frequently used for ``generic'' functions.
For example it is easy to count odd elements in a list or find the first
even number in a list:

\begin{lstlisting}[numbers=none]
scala> lst.count(odd)
5
scala> lst.find(even)
Some(2)
\end{lstlisting} 

\noindent
Recall that the return type of \code{even} and \code{odd} are booleans.
Such function are sometimes called predicates, because they determine
what should be true for an element and what false, and then performing
some operation according to this boolean. Such predicates are quite useful. 
Say you want to sort the \code{lst}-list in ascending and descending order. 
For this you can write

\begin{lstlisting}[numbers=none]
lst.sortWith(_ < _)
lst.sortWith(_ > _)
\end{lstlisting} 

\noindent where \code{sortWith} expects a predicate as argument. The
construction \code{_ < _} stands for a function that takes two arguments
and returns true when the first one is smaller than the second. You can
think of this as elegant shorthand notation for 

\begin{lstlisting}[numbers=none]
def smaller(x: Int, y: Int) : Boolean = x < y
lst.sortWith(smaller)
\end{lstlisting} 

\noindent
Say you want to find in \code{lst} the first odd number greater than 2.
For this you need to write a function that specifies exactly this
condition. To do this you can use a slight variant of the shorthand
notation above

\begin{lstlisting}[numbers=none]
scala> lst.find(n => odd(n) && n > 2)
Some(3)
\end{lstlisting} 

\noindent
Here \code{n => ...} specifies a function that takes \code{n} as
argument and uses this argument in whatever comes after the double
arrow. If you want to use this mechanism for looking for an element that
is both even and odd, then of course you out of luck.

\begin{lstlisting}[numbers=none]
scala> lst.find(n => odd(n) && even(n))
None
\end{lstlisting} 

While functions taking functions as arguments seems a rather useful
feature, the utility of returning a function might not be so clear. 
I admit the following example is a bit contrived, but believe me
sometims functions produce other functions in a very meaningful way.
Say we want to generate functions according to strings, as in

\begin{lstlisting}[numbers=none]
def mkfn(s: String) : (Int => Boolean) =
  if (s == "even") even else odd
\end{lstlisting}

\noindent
With this we can generate the required function for \code{filter}
according to a string:

\begin{lstlisting}[numbers=none]
scala> lst.filter(mkfn("even"))
List(2, 4, 6, 8, 10)
scala> lst.filter(mkfn("foo"))
List(1, 3, 5, 7, 9)
\end{lstlisting}

\noindent
As said, this is example is a bit contrived---I was not able to think
of anything simple, but for example in the Compiler module next year I
show a compilation functions that needs to generate functions as
intermediate result. Anyway, notice the interesting type we had to
annotate to \code{mkfn}. Types of Scala are described next.


\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} (see coursework). 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 remainder 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
\emph{return type} needs to be of type \code{(Int, Int)}. Incidentally,
this is also the \emph{input type} of this function. For this notice
\code{quo_rem} 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}

\noindent
This uses another special type-constructor, written as the arrow
\code{=>}. This is sometimes also called \emph{function arrow}.  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. The type of the function generated in \code{mkfn} above, is
\code{Int => Boolean}.

Unfortunately, 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 and successors was to lift this
restriction. But in all functional programming languages,
including Scala, it is really essential to allow functions as
input argument. Above you have already seen \code{map} and
\code{foreach} which need this feature. 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 interesting 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 a \emph{side-effect}, for
example 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 a side-effect. Typical
examples are the printing functions, like \code{print}.


%%\subsection*{User-Defined Types}

% \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.

%\bigskip\noindent
%\textit{More TBD.}

%\subsection*{Coursework}


\subsection*{Scala Syntax for Java Developers}

Scala compiles to the JVM, like the Java language. Because of this,
it can re-use many libraries. Here are a few hints how some Java code
tranlsates to Scala code:\bigskip

\noindent
Variable declaration:
\begin{lstlisting}[language=Java]
Drink coke = getCoke();/*!\annotation{Java}!*/
\end{lstlisting}

\begin{lstlisting}[language=Scala]
val coke : Drink = getCoke()/*!\annotation{Scala}!*/
\end{lstlisting}

\noindent
Unit means void:

\begin{lstlisting}[language=Java]
public void output(String s) {/*!\annotation{Java}!*/
  System.out.println(s);
}
\end{lstlisting}

\begin{lstlisting}[language=Scala]
def output(s: String): Unit = println(s)/*!\annotation{Scala}!*/
\end{lstlisting}

\noindent
Type for list of Strings:

\begin{lstlisting}[language=Java]
List<String>/*!\annotation{Java}!*/
\end{lstlisting}

\begin{lstlisting}[language=Scala]
List[String]/*!\annotation{Scala}!*/
\end{lstlisting}

\noindent
String interpolations

\begin{lstlisting}[language=Java]
System.out.println("Hello, "+ first + " "+ last + "!");
/*!\annotation{Java}!*/
\end{lstlisting}

\begin{lstlisting}[language=Scala]
println(s"Hello, $first $last!")/*!\annotation{Scala}!*/
\end{lstlisting}

\noindent
Java provides syntactic some sugar when constructing anonymous functions:

\begin{lstlisting}[language=Java]
list.foreach(item -> System.out.println("* " + item));
/*!\annotation{Java}!*/
\end{lstlisting}

\noindent
In Scala, we use the \code{=>} symbol:

\begin{lstlisting}[language=Scala]
list.foreach(item => println(s"* $item"))/*!\annotation{Scala}!*/
\end{lstlisting}%$

%%new / vs case classes


\subsection*{More Info}

There is much more to Scala than I can possibly describe in
this document and teach in the lectures. 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}
\item \url{https://twitter.github.io/scala_school}
\end{itemize}
 
\noindent There is also an online 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. Also the Java
language is lately developing at lightening speed (in comparison to the past) 
taking on many
features of Scala and other languages, and it seems even it introduces
new features on its own.


Scala is deep: After many years, I still continue to learn new technique
for writing more elegant code.

%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.


\subsection*{Conclusion}

I hope you liked the short journey through the Scala language---but remember we 
like you to take on board the functional programming point of view,
rather than just learning another language. There is an interesting
blog article about Scala by a convert:

\begin{center}
\url{https://www.skedulo.com/tech-blog/technology-scala-programming/}
\end{center}  

\noindent
He makes pretty much the same arguments about functional programming and
immutability (one section is teasingly called \textit{``Where Did all
the Bugs Go?''}). If you happen to moan about all the idiotic features
of Scala, well, I guess this is part of the package according to this
quote:\bigskip

%\begin{itemize}
%\item no exceptions....there two kinds, one ``global'' exceptions, like
%out of memory (not much can be done about this by the ``individual''
%programmer); and ``local one'' open a file that might not exists - in
%the latter you do not want to use exceptions, but Options
%\end{itemize}

\begin{flushright}\it
There are only two kinds of languages: the ones people complain 
about\\ and the ones nobody uses.\smallskip\\
\mbox{}\hfill\small{}---Bjarne Stroustrup (the inventor of C++)
\end{flushright}

\end{document}

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