[fpc-pascal] How to inline CompareFunc to Sort method in generic abstract class

Sven Barth pascaldragon at googlemail.com
Sun Nov 20 12:25:51 CET 2022

Am 20.11.2022 um 02:02 schrieb Hairy Pixels:
>> On Nov 20, 2022, at 4:26 AM, Sven Barth via fpc-pascal <fpc-pascal at lists.freepascal.org> wrote:
>> // this kind of constraint that uses T does not work yet
>>    generic TList<T; Comparer: specialize TComparer<T>> = class
>>      procedure Sort;
>>    end;
> Does this mean the generic param “Comparer” has a constraint which is "specialize TComparer<T>”? I’ve not ever considered this before but I think it solvers the problem where I suggested you could use something like traits.

Yes, those are type constraints. As long as you don't use types that 
were introduced in the same parameter list then it already works. E.g.:

=== code begin ===

program Project1;
{$mode objfpc}{$H+}

   generic TGen<T> = class

   generic TTest<T: specialize TGen<LongInt>> = class

   TGenLongInt = specialize TGen<LongInt>;
   TTestLongInt = specialize TTest<TGenLongInt>;



=== code end ===

Though I wouldn't really compare that with traits.

>   What is the challenge involved in implemented that so that it hasn’t been done yet? Just curious. :)

Mainly that a temporary symtable needs to be inserted before starting to 
parse the generic parameter types that is then passed along in the 
parser until the parameters are added to the final symbol table of the 
type or routine where the generic parameters reside.

> And related since it comes to mind now, what is the reason generics inside of generics (like a generic method inside a generic class) are not supported and what challenges are involved?

The main problem is that the recording of the tokens and the use of the 
recorded tokens are managed correctly. E.g. should a generic method 
inside a generic class have it's tokens recorded when they're already 
recorded as part of the type and will be recorded again as a separate 
method when the class is specialized?


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