PROGRAMMING METHODS LABORATORY

A Scala Tutorial
for Java programmers

Version 1.2
January 13, 2009
Michel Schinz, Philipp
Haller
PROGRAMMING METHODS LABORATORY
EPFL
SWITZERLAND

2
1 Introduction
This document gives a quick introduction to the Scala language and compiler. It
is intended for people who already have some programming experience and want
an overview of what they can do with Scala. A basic knowledge of object-oriented
programming, especially in Java, is assumed.
2 A first example
As a first example, we will use the standard Hello world program. It 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:
object HelloWorld {
def main(args: Array[String]) {
println("Hello, world!")
}
}
The structure of this program should be familiar to Java programmers: it consists
of one method called 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 predefined
method println with the friendly greeting as argument. The main method
does not return a value (it is a procedure method). Therefore, it is not necessary to
declare a return type.
What is less familiar to Java programmers is the object declaration containing the
main method. Such a declaration introduces what is commonly known as a singleton
object, that is a class with a single instance. The declaration above thus declares
both a class called HelloWorld and an instance of that class, also called HelloWorld.
This instance is created on demand, the first time it is used.
The astute reader might have noticed that the main method is not declared as static
here. This is because static members (methods or fields) do not exist in Scala. Rather
than defining static members, the Scala programmer declares these members in
singleton objects.
2.1 Compiling the example
To compile the example, we use scalac, the Scala compiler. scalac 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 HelloWorld.scala, we can compile
it by issuing the following command (the greater-than sign ‘>’ represents the shell
prompt and should not be typed):
2.2 Running the example 3
> scalac HelloWorld.scala
This will generate a few class files in the current directory. One of themwill be called
HelloWorld.class, and contains a class which can be directly executed using the
scala command, as the following section shows.
2.2 Running the example
Once compiled, a Scala program can be run using the scala command. Its usage is
very similar to the java command used to run Java programs, and accepts the same
options. The above example can be executed using the following command, which
produces the expected output:
> scala classpath
. HelloWorld
Hello, world!
3 Interaction with Java
One of Scala’s strengths is that it makes it very easy to interact with Java code. All
classes from the java.lang package are imported by default, while others need to
be imported explicitly.
Let’s look at an example that demonstrates this. We want to obtain and format the
current date according to the conventions used in a specific country, say France1.
Java’s class libraries define powerful utility classes, such as Date and DateFormat.
Since Scala interoperates seemlessly with Java, there is no need to implement equivalent
classes in the Scala class library–we can simply import the classes of the corresponding
Java packages:
import java.util.{Date, Locale}
import java.text.DateFormat
import java.text.DateFormat._
object FrenchDate {
def main(args: Array[String]) {
val now = new Date
val df = getDateInstance(LONG, Locale.FRANCE)
println(df format now)
}
}
1Other regions such as the french speaking part of Switzerland use the same conventions.
4
Scala’s import statement looks very similar to Java’s equivalent, however, it is more
powerful. Multiple classes can be imported from the same package by enclosing
them in curly braces as on the first line. Another difference is that when importing
all the names of a package or class, one uses the underscore character (_) instead of
the asterisk (*). That’s because the asterisk is a valid Scala identifier (e.g. method
name), as we will see later.
The import statement on the third line therefore imports all members of the DateFormat
class. This makes the static method getDateInstance and the static field LONG directly
visible.
Inside the main method we first create an instance of Java’s Date class which by
default contains the current date. Next, we define a date format using the static
getDateInstance method that we imported previously. Finally, we print the current
date formatted according to the localized DateFormat instance. This last line shows
an interesting property of Scala’s syntax. Methods taking one argument can be used
with an infix syntax. That is, the expression
df format now
is just another, slightly less verbose way of writing the expression
df.format(now)
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 integrationwith Java, it should be noted that it is also
possible to inherit from Java classes and implement Java interfaces directly in Scala.
4 Everything is an object
Scala is a pure object-oriented language in the sense that everything is an object,
including numbers or functions. It differs from Java in that respect, since Java distinguishes
primitive types (such as boolean and int) from reference types, and does
not enable one to manipulate functions as values.
4.1 Numbers are objects
Since numbers are objects, they also have methods. And in fact, an arithmetic expression
like the following:
1 + 2 * 3 / x
consists exclusively of method calls, because it is equivalent to the following expression,
as we saw in the previous section:
4.2 Functions are objects 5
(1).+(((2).*(3))./(x))
This alsomeans that +, *, etc. are valid identifiers in Scala.
The parentheses around the numbers in the second version are necessary because
Scala’s lexer uses a longest match rule for tokens. Therefore, it would break the following
expression:
1.+(2)
into the tokens 1., +, and 2. The reason that this tokenization is chosen is because 1.
is a longer valid match than 1. The token 1. is interpreted as the literal 1.0, making
it a Double rather than an Int. Writing the expression as:
(1).+(2)
prevents 1 from being interpreted as a Double.
4.2 Functions 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
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 oncePerSecond, and it gets
a call-back function as argument. The type of this function is written () => unit
and is the type of all functions which take no arguments and return nothing (the
type unit is similar to void in C/C++). 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 “time flies like an arrow”
every second.
object Timer {
def oncePerSecond(callback: () => unit) {
while (true) { callback(); Thread sleep 1000 }
}
def timeFlies() {
println("time flies like an arrow...")
}
6
def main(args: Array[String]) {
oncePerSecond(timeFlies)
}
}
Note that in order to print the string, we used predefined method println instead
of using the one fromSystem.out.
4.2.1 Anonymous functions
While this program is easy to understand, it can be refined a bit. First of all, notice
that the function timeFlies is only defined in order to be passed later to the
oncePerSecond function. Having to name 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 oncePerSecond. This is possible in Scala using anonymous
functions, which are exactly that: functions without a name. The revised version
of our timer program using an anonymous function instead of timeFlies looks
like that:
object TimerAnonymous {
def oncePerSecond(callback: () => unit) {
while (true) { callback(); Thread sleep 1000 }
}
def main(args: Array[String]) {
oncePerSecond(() =>
println("time flies like an arrow..."))
}
}
The presence of an anonymous function in this example is revealed by the right arrow
‘=>’ 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 timeFlies above.
5 Classes
As we have seen above, Scala is an object-oriented language, and as such it has a
concept of class.2 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.
class Complex(real: double, imaginary: double) {
2For 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.
5.1 Methods without arguments 7
def re() = real
def im() = imaginary
}
This complex class takes two arguments, which are the real and imaginary part
of the complex. These arguments must be passed when creating an instance of
class Complex, as follows: new Complex(1.5, 2.3). The class contains two methods,
called re and 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 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 to be easy to deduce from the context,
and see if the compiler agrees. After some time, the programmer should get a
good feeling about when to omit types, and when to specify them explicitly.
5.1 Methods without arguments
A small problem of the methods re and 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:
object ComplexNumbers {
def main(args: Array[String]) {
val c = new Complex(1.2, 3.4)
println("imaginary part: " + c.im())
}
}
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 them as methods without arguments. Such methods 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 Complex class can be rewritten
as follows:
class Complex(real: double, imaginary: double) {
def re = real
def im = imaginary
}
8
5.2 Inheritance and overriding
All classes in Scala inherit from a super-class. When no super-class is specified, as
in the Complex example of previous section, 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
override modifier, in order to avoid accidental overriding. As an example, our
Complex class can be augmented with a redefinition of the toString method inherited
from Object.
class Complex(real: double, imaginary: double) {
def re = real
def im = imaginary
override def toString() =
"" + re + (if (im < 0) "" else "+") + im + "i"
}
6 Case classes and pattern matching
A kind of data structure that often appears in programs is the tree. For example, interpreters
and compilers usually represent programs internally as trees; XML documents
are trees; and several kinds of containers are based on trees, like red-black
trees.
We will now examine how such trees are represented and manipulated in Scala
through a small calculator program. The aim of this program is to manipulate very
simple arithmetic expressions composed of sums, integer constants and variables.
Two examples of such expressions are 1Å2 and (x Åx)Å(7Å y).
We first have to decide on a representation for such expressions. The most natural
one is the tree, where nodes are operations (here, the addition) and leaves are values
(here constants or variables).
In Java, such a tree would be represented using an abstract super-class for the trees,
and one concrete sub-class per node or leaf. In a functional programming language,
one would use an algebraic data-type for the same purpose. Scala provides the concept
of case classes which is somewhat in between the two. Here is how they can be
used to define the type of the trees for our example:
abstract class Tree
case class Sum(l: Tree, r: Tree) extends Tree
case class Var(n: String) extends Tree
case class Const(v: int) extends Tree
The fact that classes Sum, Var and Const are declared as case classes means that they
differ fromstandard classes in several respects:
6 Case classes and pattern matching 9
• the new keyword is not mandatory to create instances of these classes (i.e. one
can write Const(5) instead of new Const(5)),
• getter functions are automatically defined for the constructor parameters (i.e.
it is possible to get the value of the v constructor parameter of some instance
c of class Const just by writing c.v),
• default definitions for methods equals and hashCode are provided,whichwork
on the structure of the instances and not on their identity,
• a default definition for method toString is provided, and prints the value in a
“source form” (e.g. the tree for expression xÅ1 prints as Sum(Var(x),Const(1))),
• instances of these classes can be decomposed through pattern matching as
we will see below.
Now that we have defined the data-type to represent our arithmetic expressions, we
can start defining operations to manipulate them. We will start with a function to
evaluate an expression in some environment. The aim of the environment is to give
values to variables. For example, the expression x Å1 evaluated in an environment
which associates the value 5 to variable x, written {x !5}, gives 6 as result.
We therefore have to find a way to represent environments. We could of course
use some associative data-structure like a hash table, but we can also directly use
functions! An environment is really nothing more than a function which associates
a value to a (variable) name. The environment {x !5} given above can simply be
written as follows in Scala:
{ case "x" => 5 }
This notation defines a function which, when given the string "x" as argument, returns
the integer 5, and fails with an exception otherwise.
Before writing the evaluation function, let us give a name to the type of the environments.
We could of course always use the type String => int for environments,
but it simplifies the program if we introduce a name for this type, and makes future
changes easier. This is accomplished in Scala with the following notation:
type Environment = String => int
Fromthen on, the type Environment can be used as an alias of the type of functions
from String to int.
We can now give the definition of the evaluation function. Conceptually, it is very
simple: the value of a sum of two expressions is simply the sumof the value of these
expressions; the value of a variable is obtained directly from the environment; and
the value of a constant is the constant itself. Expressing this in Scala is not more
difficult:
def eval(t: Tree, env: Environment): int = t match {
10
case Sum(l, r) => eval(l, env) + eval(r, env)
case Var(n) => env(n)
case Const(v) => v
}
This evaluation function works by performing pattern matching on the tree t. Intuitively,
themeaning of the above definition should be clear:
1. it first checks if the tree t is a Sum, and if it is, it binds the left sub-tree to a new
variable called l and the right sub-tree to a variable called r, and then proceeds
with the evaluation of the expression following the arrow; this expression
can (and does) make use of the variables bound by the pattern appearing
on the left of the arrow, i.e. l and r,
2. if the first check does not succeed, that is if the tree is not a Sum, it goes on and
checks if t is a Var; if it is, it binds the name contained in the Var node to a
variable n and proceeds with the right-hand expression,
3. if the second check also fails, that is if t is neither a Sum nor a Var, it checks
if it is a Const, and if it is, it binds the value contained in the Const node to a
variable v and proceeds with the right-hand side,
4. finally, if all checks fail, an exception is raised to signal the failure of the pattern
matching expression; this could happen here only if more sub-classes of
Tree were declared.
We see that the basic idea of pattern matching is to attempt to match a value to a
series of patterns, and as soon as a pattern matches, extract and name various parts
of the value, to finally evaluate some code which typically makes use of these named
parts.
A seasoned object-oriented programmer might wonder why we did not define eval
as a method of class Tree and its subclasses. We could have done it actually, since
Scala allowsmethod definitions in case classes just like in normal classes. Deciding
whether to use pattern matching or methods is therefore a matter of taste, but it
also has important implications on extensibility:
• when using methods, it is easy to add a new kind of node as this can be done
just by defining the sub-class of Tree for it; on the other hand, adding a new
operation to manipulate the tree is tedious, as it requires modifications to all
sub-classes of Tree,
• when using pattern matching, the situation is reversed: adding a new kind of
node requires the modification of all functions which do patternmatching on
the tree, to take the new node into account; on the other hand, adding a new
operation is easy, by just defining it as an independent function.
6 Case classes and pattern matching 11
To explore pattern matching further, let us define another operation on arithmetic
expressions: symbolic derivation. The reader might remember the following rules
regarding this operation:
1. the derivative of a sum is the sumof the derivatives,
2. the derivative of some variable v is one if v is the variable relative to which the
derivation takes place, and zero otherwise,
3. the derivative of a constant is zero.
These rules can be translated almost literally into Scala code, to obtain the following
definition:
def derive(t: Tree, v: String): Tree = t match {
case Sum(l, r) => Sum(derive(l, v), derive(r, v))
case Var(n) if (v == n) => Const(1)
case _ => Const(0)
}
This function introduces two new concepts related to pattern matching. First of all,
the case expression for variables has a guard, an expression following the if keyword.
This guard prevents pattern matching from succeeding unless its expression
is true. Here it is used to make sure that we return the constant 1 only if the name of
the variable being derived is the same as the derivation variable v. The second new
feature of patternmatching used here is the wild-card, written _, which is a pattern
matching any value, without giving it a name.
We did not explore the whole power of pattern matching yet, but we will stop here
in order to keep this document short. We still want to see how the two functions
above perform on a real example. For that purpose, let’s write a simple main function
which performs several operations on the expression (x Å x)Å(7Å y): it first
computes its value in the environment {x !5, y !7}, then computes its derivative
relative to x and then y.
def main(args: Array[String]) {
val exp: Tree = Sum(Sum(Var("x"),Var("x")),Sum(Const(7),Var("y")))
val env: Environment = { case "x" => 5 case "y" => 7 }
println("Expression: " + exp)
println("Evaluation with x=5, y=7: " + eval(exp, env))
println("Derivative relative to x:\n " + derive(exp, "x"))
println("Derivative relative to y:\n " + derive(exp, "y"))
}
Executing this program, we get the expected output:
Expression: Sum(Sum(Var(x),Var(x)),Sum(Const(7),Var(y)))
Evaluation with x=5, y=7: 24
12
Derivative relative to x:
Sum(Sum(Const(1),Const(1)),Sum(Const(0),Const(0)))
Derivative relative to y:
Sum(Sum(Const(0),Const(0)),Sum(Const(0),Const(1)))
By examining the output, we see that the result of the derivative should be simplified
before being presented to the user. Defining a basic simplification function
using pattern matching is an interesting (but surprisingly tricky) problem, left as an
exercise for the reader.
7 Mixins
Apart from inheriting code from a super-class, a Scala class can also import code
from one or several mixins.
Maybe the easiest way for a Java programmer to understand what mixins are is to
view them as interfaces which can also contain code. In Scala, when a class inherits
from a mixin, it implements that mixin’s interface, and inherits all the code
contained in the mixin.
To see the usefulness of mixins, let’s look at a classical example: ordered objects.
It is often useful to be able to compare objects of a given class among themselves,
for example to sort them. In Java, objects which are comparable implement the
Comparable interface. In Scala, we can do a bit better than in Java by defining our
equivalent of Comparable as a mixin, which we will call Ord.
When comparing objects, six different predicates can be useful: smaller, smaller
or equal, equal, not equal, greater or equal, and greater. However, defining all of
them is fastidious, especially since four out of these six can be expressed using the
remaining two. That is, given the equal and smaller predicates (for example), one
can express the other ones. In Scala, all these observations can be nicely captured
by the following mixin declaration:
trait Ord {
def < (that: Any): boolean
def <=(that: Any): boolean = (this < that) || (this == that)
def > (that: Any): boolean = !(this <= that)
def >=(that: Any): boolean = !(this < that)
}
This definition both creates a new type called Ord, which plays the same role as
Java’s Comparable interface, and default implementations of three predicates in terms
of a fourth, abstract one. The predicates for equality and inequality do not appear
here since they are by default present in all objects.
The type Any which is used above is the type which is a super-type of all other types
in Scala. It can be seen as a more general version of Java’s Object type, since it is
7 Mixins 13
also a super-type of basic types like int, float, etc.
To make objects of a class comparable, it is therefore sufficient to define the predicateswhich
test equality and inferiority, and mix in the Ord class above. As an example,
let’s define a Date class representing dates in the Gregorian calendar. Such dates
are composed of a day, a month and a year, which we will all represent as integers.
We therefore start the definition of the Date class as follows:
class Date(y: int, m: int, d: int) extends Ord {
def year = y
def month = m
def day = d
override def toString(): String = year + ""
+ month + ""
+ day
The important part here is the extends Ord declarationwhich follows the classname
and parameters. It declares that the Date class inherits from the Ord class as amixin.
Then, we redefine the equals method, inherited from Object, so that it correctly
compares dates by comparing their individual fields. The default implementation
of equals is not usable, because as in Java it compares object physically. We arrive
at the following definition:
override def equals(that: Any): boolean =
that.isInstanceOf[Date] && {
val o = that.asInstanceOf[Date]
o.day == day && o.month == month && o.year == year
}
This methodmakes use of the predefined methods isInstanceOf and asInstanceOf.
The first one, isInstanceOf, corresponds to Java’s instanceof operator, and returns
true if and only if the object on which it is applied is an instance of the given type.
The second one, asInstanceOf, corresponds to Java’s cast operator: If the object is
an instance of the given type, it is viewed as such, otherwise a ClassCastException
is thrown.
Finally, the last method to define is the predicate which tests for inferiority, as follows.
It makes use of another predefined method, error, which throws an exception
with the given error message.
def <(that: Any): boolean = {
if (!that.isInstanceOf[Date])
error("cannot compare " + that + " and a Date")
val o = that.asInstanceOf[Date]
(year < o.year) ||
(year == o.year && (month < o.month ||
(month == o.month && day < o.day)))
14
}
This completes the definition of the Date class. Instances of this class can be seen
either as dates or as comparable objects. Moreover, they all define the six comparison
predicates mentioned above: equals and < because they appear directly in the
definition of the Date class, and the others because they are inherited from the Ord
mixin.
Mixins are useful in other situations than the one shown here, of course, but discussing
their applications in length is outside the scope of this document.
8 Genericity
The last characteristic of Scala we will explore in this tutorial is genericity. Java programmers
should be well aware of the problems posed by the lack of genericity in
their language, a shortcoming which is addressed in Java 1.5.
Genericity is the ability to write code parametrized by types. For example, a programmer
writing a library for linked lists faces the problem of deciding which type
to give to the elements of the list. Since this list is meant to be used in many different
contexts, it is not possible to decide that the type of the elements has to be, say,
int. This would be completely arbitrary and overly restrictive.
Java programmers resort to using Object, which is the super-type of all objects. This
solution is however far from being ideal, since it doesn’t work for basic types (int,
long, float, etc.) and it implies that a lot of dynamic type casts have to be inserted
by the programmer.
Scala makes it possible to define generic classes (and methods) to solve this problem.
Let us examine this with an example of the simplest container class possible: a
reference, which can either be empty or point to an object of some type.
class Reference[a] {
private var contents: a = _
def set(value: a) { contents = value }
def get: a = contents
}
The class Reference is parametrized by a type, called a, which is the type of its element.
This type is used in the body of the class as the type of the contents variable,
the argument of the set method, and the return type of the get method.
The above code sample introduces variables in Scala, which should not require further
explanations. It is however interesting to see that the initial value given to that
variable is _, which represents a default value. This default value is 0 for numeric
types, false for the boolean type, () for the unit type and null for all object types.
9 Conclusion 15
To use this Reference class, one needs to specify which type to use for the type parameter
a, that is the type of the element contained by the cell. For example, to
create and use a cell holding an integer, one could write the following:
object IntegerReference {
def main(args: Array[String]) {
val cell = new Reference[Int]
cell.set(13)
println("Reference contains the half of " + (cell.get * 2))
}
}
As can be seen in that example, it is not necessary to cast the value returned by the
get method before using it as an integer. It is also not possible to store anything but
an integer in that particular cell, since it was declared as holding an integer.
9 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
Scala By Example, which contains muchmore advanced examples, and consult the
Scala Language Specification when needed.