Partial application on Nth argument

[Salih Dincer]

On Wednesday, 4 December 2024 at 08:50:21 UTC, axricard wrote:
> Hello
>
> I believe std.functional.partial can only apply to the first 
> argument.
> Is there an equivalent for the Nth argument ?
>
> Something like :
>
> ``` D
> int fun(int a, int b) { return a - b; }
>
> // create a function with the argument n°1 being equal to 5
> // fun5 = fun(a, 5);
> alias fun5 = partialN!(fun, 1, 5);
> writeln(fun5(7)); // print 2
> ```

Why does 'alias' take 3 parameters? Maybe I'm misunderstanding, 
but this is what you want:

```d
template partialN(alias func, int idx, args...)
{
   enum error = "Missing argument!";
   static assert(idx < args.length, error);

   auto partialN(T)(T first)
   {
     return func(first, args[idx]);
   }
} unittest {
   auto sum = (int a, int b) => a + b;

   alias sum11= partialN!(sum, 1, 10, 11, 12);
   assert(sum11(9) == 20);

   alias sum12= partialN!(sum, 2, 10, 11, 12);
   assert(sum12(9) == 21);

   auto and = (bool a, bool b) => a & b;
   alias and11= partialN!(and, 1, false, true, false);
   assert(and11(true));
}
```

SDB at 79