Please explain why this error happens in the following code:

import std.algorithm;

struct S
{
  void foo ()
  {
    int f1 (int a) { return conv(a); }
    int delegate (int) f2 = &conv;

    int[] x = [1, 2, 3];
    x.map!conv;    // ERROR
    x.map!f1;      // fine
    x.map!f2;      // also fine
  }

  int conv (int a)
  {
    return a+1;
  }
}

--- compile output:

/usr/include/d/std/algorithm.d(404): Error: this for conv needs to be type S not type MapResult!(conv, int[])
/usr/include/d/std/algorithm.d(438): Error: this for conv needs to be type S not type MapResult!(conv, int[])
/usr/include/d/std/algorithm.d(450): Error: this for conv needs to be type S not type MapResult!(conv, int[])
/usr/include/d/std/algorithm.d(390): Error: template instance std.algorithm.MapResult!(conv, int[]) error instantiating
/home/peter/proggen/goliza.reduced/gtp.d(12):        instantiated from here: map!(int[])
/home/peter/proggen/goliza.reduced/gtp.d(12): Error: template instance std.algorithm.map!(conv).map!(int[]) error instantiating

---

Thanks,
-Peter

On Saturday, 29 June 2013 at 19:44:00 UTC, Peter Neubauer wrote:
> Please explain why this error happens in the following code:
>
> import std.algorithm;
>
> struct S
> {
>   void foo ()
>   {
>     int f1 (int a) { return conv(a); }
>     int delegate (int) f2 = &conv;
>
>     int[] x = [1, 2, 3];
>     x.map!conv;    // ERROR
>     x.map!f1;      // fine
>     x.map!f2;      // also fine
>   }
>
>   int conv (int a)
>   {
>     return a+1;
>   }
> }
>
> --- compile output:
>
> /usr/include/d/std/algorithm.d(404): Error: this for conv needs to be type S not type MapResult!(conv, int[])
> /usr/include/d/std/algorithm.d(438): Error: this for conv needs to be type S not type MapResult!(conv, int[])
> /usr/include/d/std/algorithm.d(450): Error: this for conv needs to be type S not type MapResult!(conv, int[])
> /usr/include/d/std/algorithm.d(390): Error: template instance std.algorithm.MapResult!(conv, int[]) error instantiating
> /home/peter/proggen/goliza.reduced/gtp.d(12):        instantiated from here: map!(int[])
> /home/peter/proggen/goliza.reduced/gtp.d(12): Error: template instance std.algorithm.map!(conv).map!(int[]) error instantiating
>
> ---
>
> Thanks,
> -Peter

I am not sure if this behavior is intended. Since "conv" is a instance method of S, it has an implicit "this" which must be of type S. I suspect that the map template tries to invoke conv in the context of MapResult. I hope template masters can give you a more complete explanation though.