More on ParaSail interfaces

Here is the ParaSail example of an Array interface from the last installment:

 interface Array < Element_Type is Assignable<>;
                    Index_Type is Discrete<>>
 is
   function First(Arr : Array) -> Index_Type;
   function Last(Arr : Array) -> Index_Type;
   function Element(Arr : ref Array;
     Index : Index_Type {Index in First(Arr)..Last(Arr)})
       -> ref Element_Type;
   function Create_Array(First, Last : Index_Type)
       -> Result:Array
           {First(Result) = First & Last(Result) = Last};
   ...
 end interface Array;

To make indexing into an Array look more familiar, we might want to use the "[]" operator instead of the function Element:

  operator "[]"(Arr : ref Array;
     Index : Index_Type {Index in First(Arr)..Last(Arr)})
       -> ref Element_Type;

Now we could use this interface roughly as follows:

   var Names: Array<String, Student_ID> := 
     Create_Array(First => First_Student_ID,
                  Last => Last_Student_ID);
   Names[ID_Of_Mike] := "Mike";

This kind of thing should look pretty familiar.

A few things to note:  There is no separate notion of a constructor.  A function that returns an object of the type defined by the enclosing interface is effectively a constructor.   A function that returns a ref is not a constructor, but may be used on the left-hand side of an assignment, provided the ref parameter(s) to the function can be used on the left-hand side.  Parameters are by default read-only, but ref parameters can be updated.  The result of a function is specified after a "->".  If the function has multiple results, they are enclosed in parentheses in the same way as the (input) parameters:

  function Two_Outputs(X : Input_Type) -> 
         (Y : Output_Type1; Z : Output_Type2);

Once we start talking about statements, we will see that in ParaSail, as in C++ and Java, declarations and executable statements may be interspersed.  All declarations start with a reserved word, such as var, const, interface, subtype, etc. so they are easy to distinguish from assignment statements, procedure calls, etc.

Next time we will discuss how a class implements an interface.