[fpc-pascal] Working with anonymous functions

[Sven Barth]

Am 10.03.2023 um 17:34 schrieb Thomas Kurz via fpc-pascal:
> Dear all,
>
> I'm testing FPC-trunk and I'm especially interested in using anonymous functions because they should simplify working with TThread.Synchronize and others a lot.
>
> I stumbled upon an issue which I'd like to post here first because I am not sure whether it's a bug or just a wrong usage on my side.
>
> Here's a small example:
>
>
> program project1;
>
> {$modeswitch anonymousfunctions}
> {$modeswitch functionreferences}
>
> uses Classes, EventLog, SysUtils;
>
> type
>    TTest = class
>      FOnLog: TLogMessageEvent;
>      procedure CallOnLog (AEventType: TEventType; AMessage: String);
>    end;
>
> procedure TTest.CallOnLog (AEventType: TEventType; AMessage: String);
>
> var
>    r: reference to TThreadMethod = nil;

/That/ should not compile. The correct syntax is "reference to function 
..." or "reference to procedure ...".

And the type you're looking for is "TThreadProcedure" which is declared 
as "reference to procedure".

>    m: TThreadMethod = nil;
>
> begin
>    // This doesn't work because the anonymous procedure is
>    // capturing local symbols. I understand that this doesn't work.
>    m := procedure
>      begin
>        FOnLog (Self, AEventType, AMessage);
>      end;
>
>    // As far as I have understood
>    // https://forum.lazarus.freepascal.org/index.php/topic,59468.0.html
>    // the solution should be to use a "reference to".
>    // In fact, assignment works as expected:
>    r := procedure
>      begin
>        FOnLog (Self, AEventType, AMessage);
>      end;
>
>    // But I seem not to have any change to pass the reference.
>    // Both versions fail:
>    TThread.ForceQueue (NIL, r());
>    TThread.ForceQueue (NIL, @r);
> end;
>
> begin
> end.
>
>
> As you can see, the goal is to call TThread.ForceQueue. Of course, the easiest solution was having an overloaded "TThread.ForceQueue (TThread, TThreadProcedure)" as is the case with Delphi. I have already posted a feature request on Github.

I've added the overload.

>
> But I don't know why I can't use a "reference to TThreadMethod". If I understand the feature announcement correctly, this should work, shouldn't it?

That declaration is nonsense, so no, that should definitely not work.

Regards,
Sven