Re: newbie question

["Nick Roberts" <Nick.Roberts@dial.pipex.com>]

Crucially, one of the things you can pass as a generic parameter into a
generic unit is a procedure or function.  Furthermore, the parameter types
of the parameters (and return types) of these can be other generic
parameters (which must appear beforehand).

Thus one might declare:

   generic
      type Element_Type is private;
      type Index_Type is (<>);
      type Array_Type is array (Index_Type) of Element_Type;
      with procedure Swap (Item1, Item_2: in out Element_Type) is <>;
      with function "<=" (Left, Right: in Element_Type) return Boolean is
<>;

   procedure My_Sort (Object: in out Array_Type; From, To: Index_Type);

and complete this declaration with a body:

   procedure My_Sort (Object: in out Array_Type; From, To: Index_Type) is
      ...
   end My_Sort;

within which you can refer to the generic parameters (almost) as if they
were ordinary examples of the corresponding kinds of entity.

The 'is <>' at the end of the procedure and function generic parameters
allow an instantiation of the generic procedure (My_Sort) to be omitted, if
there is a suitable procedure or function already in existence which has the
same name ('Swap' or the operator "<=").  A lot of types already have "<="
defined for them (the predefined types Integer, Float, and String, for
example).

The generic procedure can then be 'instantiated' to create a specific
procedure which can actually be called.  For example, supposing we had:

   type Day is (Monday, ..., Sunday);

   type Earnings is array (Day) of Float;

   procedure Swap (F1, F2: in out Float) is
      F3: constant Float := F1;
   begin
      F1 := F2;
      F2 := F3;
   end Swap;

we could then declare the following instantiation:

   procedure Sort_Earnings is new My_Sort(Float,Day,Earnings);

The procedure Sort_Earnings will sort an array of the type Earnings.  The
generic parameters 'Swap' and "<=" did not have to explicitly passed in the
instantiation, because they already existed, and could be 'assumed'.

I hope this is not too complicated to take in in one gulp!  A good textbook,
tutor, friendly local expert, or online tutorial will repay your study.
Best of luck.

-------------------------------------
Nick Roberts
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