Smarter Generics

["Frank J. Lhota" <NOSPAM.lhota.adarose@verizon.net>]

The recent thread on "A Simple Ada Problem" got me thinking about a
limitation of Ada generics. Generic are a great way of generalizing an
implementation strategy for a particular problem, but generics cannot (or
cannot easily) select one of several implementation strategies based on the
generic parameters.

Take the following simple example: Assume that I want to implement stacks of
limited length, e.g. a stack that I can push or pop elements of a given
type, but with a limit on how many elements will be on the stack at one
time:

    generic
        type Stack_Element is private;
        Max_Length : in Positive;
    package Limited_Stacks is

        type Stack_Type is limited private;
        Stack_Overflow, Stack_Underflow : exception;

        procedure Push ( Item : in Stack_Element; Onto : in out
Stack_Type );
        procedure Pop ( Item : out Stack_Element; Onto : in out
Stack_Type );

    private
        ...
    end Limited_Stacks;

Two possible implementation strategies:

1.    We could implement the stack as an array of Max_Length elements, along
with a component indicating the current length.
2.    We could maintain an adjustable list of pointers to elements allocated
from the heap.

Obviously, implementation 1 would be faster and more convenient if
Max_Length * Stack_Element'Size is relatively small. Implementation 2
involves additional overhead, but can handle cases where stack elements can
vary quite a bit in size.

What would be great is if there were some way to write Limited_Stacks so
that the implementation strategy could be chosen based on the generic
parameters, so that we could do something like this:

    if Max_Length * Stack_Element'Size <= Some_Threshold then
        -- Static memory allocation should be fine
        Implement Limited_Stacks using strategy 1;
    else
        -- Size could be a problem, better go with pointers.
        Implement Limited_Stacks using strategy 2;
    end if;

AFAIK neither Ada generics nor C++ templates have this level of
intelligence. Would smarter generics be a worthwhile addition to Ada, and if
so, what form should it take?