Experimenting with the Magda Language

mixin HelloWorld of Object =
new Object MainMatter()
begin
‘‘Hello world’’.String.print();
end;
end;
//
(new HelloWorld []).HelloWorld.MainMatter();
Figure 2.1: Minimal program example
begins with the keyword new which indicates the introduction of a new method identi-
er , as opposed to method redenition , and abstract methods, both described later on.
The main instruction starts with the creation of an object from mixin HelloWorld,
using the new HelloWorld[] expression. Then it calls the method MainMatter on the
object, that, in turn, calls the method print, declared in mixin String. In Magda,
String is a built-in mixin and contains methods like print and add (which concate-
nates two strings). Similarly, Magda contains other built-in mixins like Integer and
Boolean. The Boolean mixin cannot be used in object creation expressions. Then, the
only way to obtain a Boolean value is to use one of the Boolean constants: true and
false. Since booleans are object values, null is also the default value for Boolean.
Moreover, Magda features two control instructions: a conditional instruction if and a
loop instruction while.
In each method call, the method name is prexed with the name of the mixin in
which it was introduced. This is to enforce hygiene of identiers in order to avoid
accidental clashes.
In the current version of Magda all methods are visible. This results in a similar
behavior as the one of public methods in Java.
The following sections will show on object usage, composability of inheritance de-
pendecies, abstract and virtual method declarations. About the initialization protocol
behaviour see the dedicated Section 2.3.2.
2.2.1 Object elds and local variables
All elds of an object are declared, with a name and a type, in the mixins from which
the object is created. The type of a eld is a sequence of mixin names. The value of
each eld can be either the null value or an object value, that is, a reference to an
object. For simplicity, we have chosen that each eld in a Magda object can be only
accessed by methods of that object, via an invocation of the form this.fieldId. As
a result we obtain a behavior close to the one of protected elds in other languages.
Furthermore, similarly to method identiers, all elds are prexed with the name of
the mixin in which the elds have been declared.
In the example in Figure 2.2 there is a declaration of a mixin Point2D containing
two eld declarations, x and y.
As in MainClass.MainMatter method, variable declarations are in the header of
the method, after the signature, before the keyword begin, The type of a variable is a
sequence of mixin names; in this example, variable p1 has type Point2D. Similarly to
elds, local variables can be assigned with object values.
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