taskPool.reduce vs algorithm.reduce

[Timoses]

On Wednesday, 11 July 2018 at 08:31:30 UTC, Dorian Haglund wrote:
> Hi.
>
> I'm trying to use taskPool.reduce with a delegate, for example:
>
> import std.parallelism;
>
> int main(string[] args)
> {
>   int f(int a, int b)
>   {
>     if (args.length > 1)
>       return a+b;
>     else
>       return a-b;
>   }
>
>   auto res = taskPool.reduce!f([1, 2, 3]);
>
>   return 0;
> }
>
> But it fails to compile (with gdc 8.1.0, dmd v2.081) 
> complaining that
>
> template instance reduce!(f) cannot use local 'f' as parameter 
> to non-global template reduce(functions...)
>
> The snippet above compiles with the reduce function from 
> std.algorithm.
>
> Is there a way to make the code compile with taskPool.reduce ?
> (I don't want to write two functions and choosing one depending 
> on args.length)
>
> Why the interface difference between std.algorithm's reduce and 
> taskPool.reduce ?
>
> Best regards,
>
> Dorian

As the error message says taskPool.reduce is a non-global 
template. It's embedded in a taskPool struct. I can't say what 
the reason is that a delegate cannot be used with such a 
template. I'd be interested in hearing what the reason is.
(See Paul's reply).

I'm trying to trick around it, but can't get this to work...

https://run.dlang.io/is/EGbtuq

import std.parallelism;

int main(string[] args)
{
     static int f(bool cond)(int a, int b)
     {
         static if (cond)
             return a+b;
         else
             return a-b;
     }

     template getF(alias func)
     {
         auto getF(T)(T arg)
         {
             if (args.length > 1)
                 return func!(f!true)(arg); // line 18
             else
                 return func!(f!false)(arg); // line 20
         }
     }

     auto res = getF!(taskPool.reduce)([1,2,3]);

     return 0;
}


onlineapp.d(18): Error: need this for reduce of type @system 
int(int[] _param_0)
onlineapp.d(20): Error: need this for reduce of type @system 
int(int[] _param_0)