The Green Language

class Container public: abstract proc add( x : integer ) abstract proc get() : integerproc get() : integer proc empty() : boolean begin return getSize() == 0; end proc full() : boolean begin return getSize() == getMaxSize(); end abstract proc getSize() : integerproc getSize() : integer abstract proc getMaxSize() : integer; endproc getMaxSize() : integer; end Figura 3.4: Class Container for storing objects proc cast( ch : char ) : integer assert after result >= 0 and result <= 127; end 3.6 Abstract Classes A class in which some method bodies may not defined is called abstract. The class declaration is preceded by the keyword abstract and so are the methods not fully defined. Although the body of a method (its local variables and statements) may not be specified, its header should. As an example, the class Container of Figure 3.4 is abstract and the method bodies of put, get, getSize, and getMaxSize are missing. To fully define abstract classes we need to specify some features and restrictions, which is made below. • constructors, defined in Chapter 4, cannot be abstract; • some methods and instance variables may be fully defined and the methods may use other class methods; • an abstract class may not have any abstract method. It still is an abstract class as long as the keyword abstract precedes the word class in the class declaration; • constructors may be defined although an object of an abstract class cannot be created — abstract classes are made only to play the role of superclasses and as a design specification. 20 CAPÍTULO 3. CLASSES AND METHODS A variable whose type is an abstract class can be declared and it can refer to objects of subclasses of the abstract class; • all abstract methods must be in the public or subclass section. See Section 5.4 for the definition of subclass section. 3.7 Why Green Does Not Support Operator Overloading ? Operator overloading is the use of operators of the basic classes (+, *, <=, and, ...) as method names in normal classes. In general this makes the language more complex without adding no power to it. Other reasons for not supporting this feature are discussed next. When one defines a method proc +(other : T) : T in a class T, we assume + has the same semantics as the basic-class operator “+”. That is, “+” is free from side effects. The evaluation of “x + y” does not change the value of x or y if they are integers. It is tempting to introduce a slightly change to the semantics of a method proc +(other : Matrix) : Matrix of class Matrix in such a way the return value is self then saving the creation of a large Matrix object that would be returned by “+”. Parameter other would be added to self by Matrix::x then producing a side effect. This is the reason why normal classes cannot declare operators as methods. Besides the side-effect problem (semantics differences), there is another related to this: there is no simple way a method can emulate some operators like and and or of class boolean. These methods have short-circuit evaluation. This means an expression i < n and v[i] > 0 evaluates to false if i >= n, regardless of the result of v[i] > 0. Suppose operator and was added to a class Correct and ok is a variable of this class. The expression ok and v[i] > 0 would be equivalent to ok.and( v[i] > 0 ) and should return false if ok if false, without evaluating “v[i] > 0”. But by the semantics of method call the real parameter is always evaluated. For short, Green does not support operator overloading because of the semantic differences between basic classes and reference classes and because of the impossibility of simulating some operators. 3.8 Expanded Variables An instance or local variable may be declared as var clock : @Clock; to mean it obeys value instead of reference semantics, like the basic classes. The declaration alone allocates memory for the object, which need not to be dynamically allocated. Variables declared with @ are constants — it is a compiler error to use them in the left-hand side of an assignment. These @-variables correspond to variables of expanded classes in Eiffel or to non-pointer variables in C++. They are called expanded variables in Green. 3.8. EXPANDED VARIABLES 21 An expanded variable or @-variable should be initiated by sending to it an init message: clock.init(13, 52); This code replaces a call to method new of class Clock that would be made if clock were a regular variable: clock = Clock.new(13, 52); Messages init can only be sent to expanded variables. That will never cause an error because an expanded variable will always refer to an object of its declared class — an expanded variable cannot receive objects in assignments. Then, in particular, they cannot receive objects of subclasses which do not have a particular init method. If clock were a regular variable and “clock.init(13, 52)” were legal, a runtime type error could occur. Variable clock could refer to an object whose class does not define an init method. These methods never belong, by definition, to a class type, which is the set of public method signatures. Expanded variables always refer to the same object at runtime. This allows the compiler to optimize message sends to them. No runtime search for a method is necessary — a method call is made as efficient as a regular procedure call in procedural languages. There should not be any cycles in the classes of expanded variables. That is, it is illegal a class A declare an expanded variable of class B and class B declare an expanded variable of class A.