Correct way for partial generic specialization

[leledumbo]

This code:

{$mode objfpc}{$H+}

type
  generic TType1<A,B> = class end;
  generic TType2 = specialize TType1<A,String>;

begin
end.

fails:
x.pas(5,42) Error: Identifier not found "A"
x.pas(5,42) Error: Type identifier expected
x.pas(5,50) Error: Illegal expression
x.pas(7,1) Error: This type can't be a generic

but this code:

{$mode objfpc}{$H+}

type
  generic TType1<A,B> = class end;
  generic TType2 = class(specialize TType1<A,String>) end;

begin
end.

works. Does it have to be like the 2nd code for partial generic specialization?

[leledumbo]

Sorry, I miss something:

generic TType2 = ...

:">

[Sven Barth-2]

Am 16.04.2013 16:56 schrieb "leledumbo" <[hidden email]>:
>
> This code:
>
> {$mode objfpc}{$H+}
>
> type
>   generic TType1<A,B> = class end;
>   generic TType2  = specialize TType1<A,String>;
>
> begin
> end.
>
> fails:
> x.pas(5,42) Error: Identifier not found "A"
> x.pas(5,42) Error: Type identifier expected
> x.pas(5,50) Error: Illegal expression
> x.pas(7,1) Error: This type can't be a generic
>

As the compiler tells you: A is not a defined type. The compiler needs to know the complete type before it can use any type defined in there and in this case the full type is "TType1<A, String>" where both types must come from outside the type.

> but this code:
>
> {$mode objfpc}{$H+}
>
> type
>   generic TType1<A,B> = class end;
>   generic TType2  = class(specialize TType1<A,String>) end;
>
> begin
> end.
>
> works. Does it have to be like the 2nd code for partial generic
> specialization?

This must only work if you write "generic TType2<A> =...". Otherwise this is a bug.

One can argue however that the following should be valid as well (AFAIK it is not currently):

=== example begin ===

type
  generic TType2<A> = specialize TType1<A, String>;

=== example end ===

Regards,
Sven

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[leledumbo]

> This must only work if you write "generic TType2 =...". Otherwise this is a bug.

The code should have it, I missed it when copypaste-ing.

> One can argue however that the following should be valid as well (AFAIK it is not currently):

Yes, this is my original question. Let me rephrase:

Having this declaration:

type
  generic TType1<A,B> = class end;

Why the following is not valid:

  generic TType2 = specialize TType1<A,String>;

but this one is:

  generic TType2 = class(specialize TType1<A,String>) end;

?

Is it possible to do partial generic specialization without the need of inheritance?

[Sven Barth-2]

Am 17.04.2013 02:11 schrieb "leledumbo" <[hidden email]>:
> > One can argue however that the following should be valid as well (AFAIK it
> > is not currently):
>
> Yes, this is my original question. Let me rephrase:
>
> Having this declaration:
>
> type
>   generic TType1<A,B> = class end;
>
> Why the following is not valid:
>
>   generic TType2  = specialize TType1<A,String>;

You're missing the <A> again. :)

> but this one is:
>
>   generic TType2  = class(specialize TType1<A,String>) end;
>
> ?
>
> Is it possible to do partial generic specialization without the need of
> inheritance?

Not currently. It might be a useful extension however. I'll need to test what Delphi says about this...

Regards,
Sven

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[leledumbo]

> You're missing the  again. :)

*self-headcut

> Not currently. It might be a useful extension however. I'll need to test what Delphi says about this...

OK, I'll wait

[Sven Barth-2]

Am 17.04.2013 11:11, schrieb leledumbo:
>> You're missing the   again. :)
> *self-headcut

Can it be that your E-Mail client misinterprets the "<...>" (with only
one string as "value") as HTML tags? Because I can't see the "<A>" I had
written to you in your quotation.

>> Not currently. It might be a useful extension however. I'll need to test
>> what Delphi says about this...
> OK, I'll wait
So... Delphi does not support that, but I still think that this is a
valid extension of generics and can be used for all generic-able types
instead of only classes.

Regards,
Sven
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