Re: [fpc-devel] Local procedures as procedural parameter

[Michael Van Canneyt]

On Mon, 14 Mar 2005, Jonas Maebe wrote:

On 14 mrt 2005, at 09:45, Michael Van Canneyt wrote:

7. Consider the following program:

        program func;

        type tfun = function( x: real): real;

        procedure iso_fun( function f( x: real): real);
        begin
        end;

        procedure typ_fun( pf: tfun);
        begin
        end;

        procedure somefun;

                function f( x: real): real;
                begin
                        f:= x
                end;

        begin
                iso_fun( f);
                typ_fun( f); {procedural variable can't get nested routiine}
        end;

        begin
        end.

1. What happens if f would use a variable from somefun, and f is called
   when somefun is no longer executed ?

That is not possible. If you take the address of f and put it in a procedural variable, you'll get an error (just like when you attempt to assign a method to a regular procedural variable). You can only pass it as parameter to procedures declared with an ISO-style procedure/function parameter, and since its scope is limited to somefun, somefun will always be active when you can do so.

But the compiler can never guarantee this ?

It seems to me that the following is perfectly valid code :

Var
  StoredF : Function (x : real) : real

Procedure iso_fun( function f( x: real): real);
begin
  StoredF:=F;
end;

StoredF can then be called at will, even outside somefun.

When compiling this, there is no way the compiler can determine whether
f (or storedf) will be a local procedure or a global one. To solve this,
it would mean that each procedure variable would have to consist of 2
things; A pointer and a type indicator.

This would break _a lot_ of existing code. If that is the consequence, I
am heavily against introducing this possibility.

2. I see no difference whatsoever between typ_fun and iso_fun, except
   the use of an extra type, which, in my opinion, does not change
   anything to the usage or code of these functions. If one is allowed,
   the other should be allowed as well.

Well, it currently isn't in either our compiler or in Delphi. You cannot declare a function type inside a parameter list for now. Maybe this was even done on purpose to avoid clashes with ISO-style procedure parameters.

Yes, but why would one be allowed and the other not ?

Michael.

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