www.digitalmars.com         C & C++   DMDScript  

digitalmars.D.learn - taskPool.reduce vs algorithm.reduce

reply Dorian Haglund <dorian.haglund gmail.com> writes:
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
Jul 11 2018
next sibling parent Paul Backus <snarwin gmail.com> writes:
On Wednesday, 11 July 2018 at 08:31:30 UTC, Dorian Haglund wrote:

 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...)


Congratulations! You've just run into issue 5710 [1], one of D's 
most annoying known bugs.

[1]: https://issues.dlang.org/show_bug.cgi?id=5710


 The snippet above compiles with the reduce function from 
 std.algorithm.

 [...]

 Why the interface difference between std.algorithm's reduce and 
 taskPool.reduce ?


std.algorithm.reduce is a free function, so it can accept 
delegates as template parameters. taskPool.reduce is a member 
function, so it can't.
Jul 11 2018
prev sibling parent reply Timoses <timosesu gmail.com> writes:
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)
Jul 11 2018
parent Malte <no valid.mail> writes:
On Wednesday, 11 July 2018 at 10:07:33 UTC, Timoses wrote:

 On Wednesday, 11 July 2018 at 08:31:30 UTC, Dorian Haglund 
 wrote:

 [...]


 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


You can make it all global: https://run.dlang.io/is/Kf8CLC

 From my own experience: Use only parallel foreach and save 
yourself a lot of hassle. That just works.
Jul 19 2018