Re: generic with function procedure

[Shark8 <onewingedshark@gmail.com>]

On Friday, June 5, 2020 at 8:35:34 AM UTC-6, Gilbert Gosseyn wrote:
> 
> the message is: no visible subprogram matches the specification for "g"
>                            no visible subprogram matches the specification for "h"

Well, first, let's look at some generic code:
 Generic
   Type Element is private;
   Type Index   is (<>);
   Type Vector  is Array( Index range <> ) of Element;
   Zero : Element;
   with Function "+"( Left, Right : Element ) return Element is <>;
 Function Generic_Sum( Input : Vector ) return Element;
 Function Generic_Sum( Input : Vector ) return Element is
   Return Result : Element := Zero do
     For Item of Input Loop
       Result:= Result + Item;
     End Loop;
   End Return;
 End Generic_Sum;

Now, if we say something like:
  Package Example_1 is
     Type X is private;
     Type Y is range 1..7;
     Type Vector is Array(Y range <>) of X;

     Function "+"( Right : Vector ) return Element;
     Default : Constant X;
  Private
     Function Sum is new Generic_Sum(X,Y,Vector, Zero => Default, others => <>);
     Type X is range 0..15; -- 4-bits.
     Function "+"( Right : Vector ) return Element
        renames Sum;
     Default : Constant X := 0;
  End Example_1;

...doesn't work. Why?
Well, for one, the point where you're trying to instantiate Generic_Sum into Sum, even though the parameter (X) matches the formal parameter, at this point in the code the only thing we know is that it's a private type... it doesn't have a "+" function that it can see, and so it will tell you that there's nothing to match with function "+".

Swapping the "function Sum is" and "type X is" lines would solve that problem, but there *might* be another, similar one: the constant (Default) hasn't yet been assigned a value and so the instantiation might fail. (I'd have to re-read the ARM,)

In your post the line "procedure nm is new denm(f=>f1,g=>g1,h=>h1);" is analogous to the situation above, albeit slightly different: you're explicitly specifying that subprograms "g1" and "h1" are to be respectively associated with parameters "g" and "h" -- this is all well and good, and the compiler has caught two mistakes for you:
— no visible subprogram matches the specification for "g"
— no visible subprogram matches the specification for "h" 

So it's telling you that of all the functions "g1" that it sees at the point of instantiation, none match the generic's specification of "function g(j : Integer; v : Real_Vector) return Real;"
(and the same for "h".)

Now, at this point the compiler cannot do anything more: this is the realm where only the human programmer can tell what to do:
— Is it a copy and paste-error creating the functions "g" and "h"? (Were they declared in the form of "f" which is "(v : Real_Vector) return Real;" instead of "(j : Integer; v : Real_Vector) return Real;"?)
— Did you perhaps mean to make the formal parameters of denm, "g" and "h", to be "(v : Real_Vector) return Real;" instead of "(j : Integer; v : Real_Vector) return Real;"? (Perhaps being interrupted while refactoring it, and it's stuck in a form intermediate between what you had and what you want.)
— Did you forget to make a "g1" and "h1"?