[fpc-devel] Local procedures as procedural parameter

Jonas Maebe jonas at zeus.ugent.be
Mon Mar 14 10:29:47 CET 2005

On 14 mrt 2005, at 10:15, Michael Van Canneyt wrote:

> It seems to me that the following is perfectly valid code :
> Var
>   StoredF : Function (x : real) : real

This is a regular procedural variable, not an ISO-style procedural 
variable. Just like "var a: array of byte;" is a dynamic array and not 
an open array.

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

This assignment is invalid, it's the same as trying to assign a method 
to a regular procedural variable.

> 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.

That is not necessary.

>> 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 ?

In general, you cannot declare complex types inside a parameter list 
(you also can't declare new record, array or set types inside a 
parameter list). That's why you can (ab)use those things to declare 
special things, like open arrays (and ISO-style procvars).


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