Formal Packages definition limitation

Formal Packages definition limitation

[Alexander V. Konstantinou]

Consider a generic procedure Iter_Swap (swaps iterators)

generic
  with package Iterators1 is new Forward_Iterators(<>);
  with package Iterators2 is new Forward_Iterators(<>);
procedure Iter_Swap(A: Iterators1.Iterator; B: Iterators2.Iterator);

Now, this definition is too general since I would like to restrict
that 
	Iterators1.Value_Type = Iterators2.Value_Type

A reasonable way to do this (you can argue about the syntax) would be :

generic
  with package Iterators1 is new Forward_Iterators(<>);
  with package Iterators2 is new Forward_Iterators
    (Value_Type=>Iterators1.Value_Type);
procedure Iter_Swap(A: Iterators1.Iterator; B: Iterators2.Iterator);

I'd want the compiler to accept any instantiation of Forward_Iterators
with the limitation that its value type should be Iterators1.Value_Type.

Alas the reference manual defines (Sec. 12.7):

    formal_package_actual_part ::= (<>) | [generic_actual_part]

leaving no room for partial generic_actual_parts.  Any ideas why this
was not allowed ?  At present I am forced to pass two conversion
functions, but that is clearly not a good solution.

Alexander V. Konstantinou
akonstan@cs.columbia.edu
-- 
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Alexander V. Konstantinou              http://www.cs.columbia.edu/~akonstan/
akonstan@cs.columbia.edu               akonstan@acm.org