[fpc-devel] Delphi anonymous methods

[Sven Barth]

On 02.03.2013 20:03, vrt277 wrote:
> Implementation of anonymous methods was started in the branch. Author
> didn't submit anything last year and forgot commit one added file. But
> changes from branch can be useful, AFAIU parsing of anonymous functions
> without capturing of local variables was implemented.
>
> Does someone know is author available? Does he plan to continue work, or
> I can work on implementation without collaboration with him ? Or at
> least can he send missed file ?
>

I have already written the author some time ago. He has said that he 
plans to at least commit the missing files. If you would continue his 
work (in a new branch please with a complete copy of trunk...) then he 
might be relieved as well.

> It's hard to properly split this complex task into subtasks. I can
> propose 2 steps:
>    1. Parsing and generation of anonymous function. Capturing of
>    variables is forbidden.
>    2. Capturing of variables is implemented.
>
> A question for fpc team: do you agree that support of Delphi-compatible
> anonymous methods should be implemented?

At least I do, yes.

> Also there are open questions which require brainstorm:
> 1. Does Pascal needs other implementation of closures which is different
> from anonymous methods implementation?

I would say no. After all the method implementation itself stays the 
same, but the captured variables should be different. Or what does this 
print:

=== example begin ===

var
   arr: array[0..5] of reference to procedure;
   i: LongInt;
begin
   for i := Low(arr) to High(arr) do
     arr[i] := procedure
            begin
              Writeln(i);
            end;

   Writeln('Output: '); // just to ensure that the optimizer doesn't 
combine the two loops

   for i := Low(arr) to High(arr) do
     arr[i]();
end;

=== example end ===

> 2. Does Pascal needs short syntax for closures?

I would say yes, but this can be done in a seperate step

> I if someone want to discuss these questions it will be better to create
> in separate threads.

Two personal things I'd like to note as I have already planned to 
continue working on anonymous functions myself:
- add support to pass nested functions to "reference to" procvars; this 
will allow users who don't want to use the anonymous syntax to use the 
"closure" feature as well; please note that this must be done in a way 
that passing nested functions to "is nested" procvars is not broken
- (in context with the above point) couple the support for the anonymous 
functions to a new modeswitch (enabled by default in mode delphi), but 
allow "reference to" procvars in non-Delphi modes (this way you'd only 
be able to use nested functions, but later on the shorter syntax should 
be used there as well)

Regards,
Sven