From 330db276e68e7cd650a7294969dc512bd60d872c Mon Sep 17 00:00:00 2001
From: schinz <schinz@epfl.ch>
Date: Wed, 27 Aug 2003 06:52:47 +0000
Subject: - renamed tutorial.scala.tex to ScalaTutorial.s...

- renamed tutorial.scala.tex to ScalaTutorial.scala.tex
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+%% This file really contains -*- LaTeX -*- code, to be processed by
+%% the scalatex script.
+
+%% $Id$
+
+\documentclass[a4paper,12pt]{article}
+
+\usepackage{palatino}
+\usepackage{alltt}
+\usepackage{xspace}
+\usepackage{url}
+
+\newcommand{\langname}[1]{#1\xspace}
+
+\newcommand{\Scala}{\langname{Scala}}
+\newcommand{\Java}{\langname{Java}}
+
+\newcommand{\toolname}[1]{\texttt{#1}\xspace}
+
+\newcommand{\socos}{\toolname{socos}}
+\newcommand{\java}{\toolname{java}}
+
+\newcommand{\ident}[1]{\url{#1}\xspace}
+
+\begin{document}
+
+\title{An introduction to \Scala\\[.5em]\normalsize(for \Java programmers)}
+\author{Michel Schinz}
+\maketitle
+
+\section{Introduction}
+\label{sec:introduction}
+
+This document gives a quick introduction to the \Scala language and
+compiler. It is intended for people who have already some experience
+at programming and want an overview of what they can do with \Scala. A
+basic knowledge of object-oriented programming, especially in \Java,
+is assumed.
+
+\section{A first example}
+\label{sec:first-example}
+
+As a first example, we will use the standard \emph{Hello world}
+program, which is not very fascinating but makes it easy to
+demonstrate the use of the \Scala tools without knowing too much about
+the language. Here is how it looks:
+\begin{scalaprogram}{HelloWorld}
+object HelloWorld {
+  def main(args: Array[String]): unit = {
+    System.out.println("Hello, world!");
+  }
+}
+\end{scalaprogram}
+
+The structure of this program should be familiar to Java programmers:
+it consists of one method called \ident{main} which takes the command
+line arguments, an array of strings, as parameter; the body of this
+method consists of a single call to the \ident{println} method of the
+object representing the standard output, with the friendly greeting as
+argument. The \ident{main} method is declared as returning a value of
+type \ident{unit}, which for now can be seen as similar to \java's
+\ident{void} type.
+
+What should be less familiar to Java programmers is the \ident{object}
+declaration containing the \ident{main} method. Such a declaration
+introduces what is commonly known as a \emph{singleton object}, that
+is a class with a single instance. The declaration above thus declares
+both a class called \ident{HelloWorld} and an instance of that class,
+also called \ident{HelloWorld}. This instance is created lazily, the
+first time it is used.
+
+The astute reader might also have noticed that the \ident{main} method
+is not declared as \ident{static} here. This is because static members
+(methods or fields) do not exist in \Scala. Rather than define static
+members, the \Scala programmer declares these members in singleton
+objects.
+
+\subsection{Compiling the example}
+\label{sec:compiling-example}
+
+To compile the example, we need to use \socos, the \Scala compiler.
+\socos works like most compilers: it takes a source file as argument,
+maybe some options, and produces one or several object files. The
+object files it produces are standard \Java class files.
+
+If we save the above program in a file called
+\ident{HelloWorld.scala}, we can compile it by issuing the following
+command (the greater-than sign `\verb|>|' represents the shell prompt
+and should not be typed):
+\begin{verbatim}
+> socos HelloWorld.scala
+\end{verbatim}
+This will generate a few class files in the current directory, one of
+which called \ident{HelloWorld.class}. This file contains a class
+which can be directly executed using the \java command, as will be
+seen in the following section.
+
+\subsection{Running the example}
+\label{sec:running-example}
+
+Once compiled, a \Scala program can be run like a \Java program, using
+the \java command. However, a compiled \Scala program needs to access
+some support classes at run-time, which should be available through
+\Java's class path. These support classes are distributed in a JAR
+file called \url{scala.jar}, which lives in the directory
+\url{SCALA_HOME/lib}. Here \url{SCALA_HOME} is a place-holder for the
+name of the directory where the \Scala distribution was installed.
+
+The example above can therefore be executed using the command below,
+if we assume that the \Scala distribution was installed in
+\url{/usr/local}:
+\begin{verbatim}
+> java -classpath /usr/local/lib/scala.jar:. HelloWorld
+\end{verbatim}
+\scalaprogramoutput{HelloWorld}
+
+\section{Interaction with Java}
+\label{sec:inter-with-java}
+
+One of the strength of \Scala is that it makes it very easy to
+interact with \Java code. Actually, the example of the previous
+section showed this: to print the message on screen, we simply used a
+call to \Java's \ident{println} method on the (\Java) object
+\ident{System.out}.
+
+All \Java code is accessible as easily from \Scala. Of course, it is
+sometimes necessary to import classes, as one does in \Java, in order
+to be able to use them. All classes in the \ident{java.lang} packages
+are imported by default, others need to be imported explicitly.
+
+Let's look at another example to see this. The aim of this example is
+to compute and print the factorial of 100 using \Java big integers
+(i.e. the class \ident{java.math.BigInteger}), since the result does
+not fit in a \Java integer. This program looks like this:
+\begin{scalaprogram}{BigFactorial}
+object BigFactorial {
+  import java.math.BigInteger, BigInteger._;
+
+  def fact(x: BigInteger): BigInteger =
+    if (x == ZERO) ONE
+    else x multiply fact(x subtract ONE);
+
+  def main(args: Array[String]): unit =
+    System.out.println("fact(100) = "
+                       + fact(new BigInteger("100")));
+}
+\end{scalaprogram}
+
+\Scala's \ident{import} statement looks very similar to \Java's
+equivalent, but an important difference appears here: to import all
+the names of a package or class, one uses the underscore (\verb|_|)
+character instead of the asterisk (\verb|*|). This is due to the fact
+that, as we will see later, the asterisk is actually a valid \Scala
+identifier.
+
+The \ident{import} statement above therefore starts by importing the
+class \ident{java.math.BigInteger}, and then all the names it
+contains. This makes the static fields \ident{ZERO} and \ident{ONE}
+directly visible.
+
+The \ident{fact} method also shows some characteristics of \Scala's
+syntax. The first one is that the method body does not have to be
+surrounded by curly braces if it consists of a single expression.
+The second one is that methods taking one argument can be used with an
+infix syntax. That is, the expression
+\begin{verbatim}
+x subtract ONE
+\end{verbatim}
+is just another, slightly less verbose way of writing the expression
+\begin{verbatim}
+x.subtract(ONE)
+\end{verbatim}
+This might seem like a minor syntactic detail, but it has important
+consequences, one of which will be explored in the next section.
+
+To conclude this section about integration with \Java, it should be
+noted that it is also possible to inherit from \Java classes and
+implement \Java interfaces directly in \Scala.
+
+\section{Everything is an object}
+\label{sec:everything-an-object}
+
+\Scala is a pure object-oriented language in the sense that
+\emph{everything} is an object, including numbers or functions. It
+differs from \Java in that respect, since \Java distinguishes numeric
+types from objects, and does not enable one to manipulate functions as
+values.
+
+\subsection{Numbers are objects}
+\label{sec:numbers-are-objects}
+
+Since numbers are objects, they also have methods. And in fact, an
+arithmetic expression like the following:
+\begin{verbatim}
+1 + 2 * 3 / x
+\end{verbatim}
+consists exclusively of method calls, because it is equivalent to the
+following expression, as we saw in the previous section:
+\begin{verbatim}
+1.+(2.*(3./(x)))
+\end{verbatim}
+This also means that \ident{+}, \ident{*}, etc. are legal identifiers
+in \Scala.
+
+\subsection{Functions are objects}
+\label{sec:funct-are-objects}
+
+Perhaps more surprising for the \Java programmer, functions are also
+objects in \Scala. It is therefore possible to pass functions as
+arguments, to store them in variables, and to return them from other
+functions. This ability to manipulate functions as values is one of
+the cornerstone of a very interesting programming paradigm called
+\emph{functional programming}.
+
+As a very simple example of why it can be useful to use functions as
+values, let's consider a timer function whose aim is to perform some
+action every second. How do we pass it the action to perform? Quite
+logically, as a function. This very simple kind of function passing
+should be familiar to many programmers: it is often used in
+user-interface code, to register call-back functions which get called
+when some event occurs.
+
+In the following program, the timer function is called
+\ident{oncePerSecond}, and it gets a call-back function as argument.
+The type of this function is written \verb|() => unit| and is the type
+of all functions which have no arguments and return a value of type
+\ident{unit}. The main function of this program simply calls this
+timer function with a call-back which prints a sentence on the
+terminal. In other words, this program endlessly prints the sentence
+\emph{time flies like an arrow} every second.
+
+\begin{scalaprogram}{Timer}
+object Timer {
+  def oncePerSecond(callback: () => unit): unit =
+    while (true) { callback(); Thread sleep 1000 };
+
+  def timeFlies(): unit =
+    System.out.println("time flies like an arrow...");
+
+  def main(args: Array[String]): unit =
+    oncePerSecond(timeFlies);
+}
+\end{scalaprogram}
+
+\subsubsection{Anonymous functions}
+\label{sec:anonymous-functions}
+
+While this program is easy to understand, it can be refined a bit.
+First of all, notice that the function \ident{timeFlies} is only
+defined in order to be passed later to the \ident{oncePerSecond}
+function. Having to give a name to that function, which is only used
+once, might seem unnecessary, and it would in fact be nice to be able
+to construct this function just as it is passed to
+\ident{oncePerSecond}. This is possible in \Scala using
+\emph{anonymous functions}, which are exactly that: functions without
+a name. The revised version of our timer program using an anonymous
+function instead of \ident{timeFlies} looks like that:
+\begin{scalaprogram}{TimerAnonymous}
+object TimerAnonymous {
+  def oncePerSecond(callback: () => unit): unit =
+    while (true) { callback(); Thread sleep 1000 };
+
+  def main(args: Array[String]): unit =
+    oncePerSecond(() =>
+      System.out.println("time flies like an arrow..."));
+}
+\end{scalaprogram}
+The presence of an anonymous function in this example is revealed by
+the right arrow `\verb|=>|' which separates the function's argument
+list from its body. In this example, the argument list is empty, as
+witnessed by the empty pair of parenthesis on the left of the arrow.
+The body of the function is the same as the one of \ident{timeFlies}
+above.
+
+% TODO fonctions avec environnement
+
+\section{Classes}
+\label{sec:classes}
+
+As we have seen above, \Scala is an object-oriented language, and as
+such it has a concept of class.\footnote{For the sake of completeness,
+  it should be noted that some object-oriented languages do not have
+  the concept of class, but \Scala is not one of them.}
+Classes in \Scala are declared using a syntax which is close to
+\Java's syntax. One important difference is that classes in \Scala can
+have parameters. This is illustrated in the following definition of
+complex numbers.
+\begin{scalaprogram}{Complex}
+class Complex(real: double, imaginary: double) {
+  def re() = real;
+  def im() = imaginary;
+}
+\end{scalaprogram}
+This complex class takes two arguments, which are the real and
+imaginary part of the complex. It then defines two methods, called
+\ident{re} and \ident{im} which give access to these two parts.
+
+It should be noted that the return type of these two methods is not
+given explicitly. It will be inferred automatically by the compiler,
+which looks at the right-hand side of these methods and deduces that
+both return a value of type \ident{double}.
+
+The compiler is not always able to infer types like it does here, and
+there is unfortunately no simple rule to know exactly when it will be,
+and when not. In practice, this is usually not a problem since the
+compiler complains when it is not able to infer a type which was not
+given explicitly. As a simple rule, beginner \Scala programmers
+should try to omit type declarations which seem superfluous to them,
+because they are easily deduced from the context, and see whether the
+compiler agrees with them. After some time, they should get a good
+feeling about when to omit types, and when to specify them
+explicitly.
+
+\subsection{Methods without arguments}
+\label{sec:meth-wo-args}
+
+A small problem of the methods \ident{re} and \ident{im} is that, in
+order to call them, one has to put an empty pair of parenthesis after
+their name, as the following example shows:
+\begin{alltt}
+val c = new Complex(1.2, 3.4);
+System.out.println("imaginary part: " + \underline{c.im()});
+\end{alltt}
+It would be nicer to be able to access the real and imaginary parts
+like if they were fields, without putting the empty pair of
+parenthesis. This is perfectly doable in \Scala, simply by defining
+the methods as methods \emph{without arguments}. These differ from
+methods with zero arguments in that they don't have parenthesis after
+their name, neither in their definition nor in their use. Our
+\ident{Complex} class can be rewritten as follows:
+\begin{scalaprogram}{Complex2}
+class Complex(real: double, imaginary: double) {
+  def re = real;
+  def im = imaginary;
+}
+\end{scalaprogram}
+
+\subsection{Inheritance and overriding}
+\label{sec:inheritance}
+
+All classes in \Scala inherit from a super-class. When no super-class
+is specified, as in the \ident{Complex} example of previous section,
+\ident{scala.Object} is implicitly used.
+
+It is possible to override methods inherited from a super-class in
+\Scala. It is however mandatory to explicitly specify that a method
+overrides another one using the \ident{override} modifier, in order to
+avoid accidental overriding. As an example, our \ident{Complex} class
+can be augmented with a redefinition of the \ident{toString} method
+inherited from \ident{Object}.
+\begin{scalaprogram}{Complex3}
+class Complex(real: double, imaginary: double) {
+  def re = real;
+  def im = imaginary;
+  override def toString() =
+    "" + re + (if (im < 0) "-" else "+") + im + "i";
+}
+\end{scalaprogram}
+
+\section{Mixins}
+\label{sec:mixins}
+
+Apart from inheriting code from a super-class, a \Scala class can also
+import code from one or several \emph{mixins}.
+
+\section{Case classes and pattern matching}
+\label{sec:case-classes-pattern}
+
+
+\section{Genericity}
+\label{sec:genericity}
+
+
+
+\section{Conclusion}
+\label{sec:conclusion}
+
+This document gave a quick overview of the \Scala language and
+presented some basic examples. The interested reader can go on by
+reading the companion document \textit{Scala by example\/} and consult
+the \textit{Scala reference\/} when needed.
+
+\end{document}
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