Is it possible to infer a template's return type from what it's assigned to? If not is this a difficult or worthless feature to add?

OUT fun(IN, OUT)(IN value) {
    return value.to!OUT;
}

void main() {
    float a = 5.0;
    int b = fun(a);
}

On Wednesday, 11 April 2018 at 14:26:53 UTC, ixid wrote:
> Is it possible to infer a template's return type from what it's assigned to? If not is this a difficult or worthless feature to add?

Not really. The function call needs to make sense by itself:

fun(a)

needs to be a complete thing for a lot of things in the language to work. Type checking assumes it is there, inference assumes it is there, overloading assumes it s there, etc.

void foo(int);
void foo(float);

foo(fun(a)); // what happens?


So I don't say anything is impossible that isn't a paradox... but the effort level to solve all these problems would be really high for D.

On Wednesday, 11 April 2018 at 14:33:06 UTC, Adam D. Ruppe wrote:
> On Wednesday, 11 April 2018 at 14:26:53 UTC, ixid wrote:
>> Is it possible to infer a template's return type from what it's assigned to? If not is this a difficult or worthless feature to add?
>
> Not really. The function call needs to make sense by itself:
>
> fun(a)
>
> needs to be a complete thing for a lot of things in the language to work. Type checking assumes it is there, inference assumes it is there, overloading assumes it s there, etc.
>
> void foo(int);
> void foo(float);
>
> foo(fun(a)); // what happens?
>
>
> So I don't say anything is impossible that isn't a paradox... but the effort level to solve all these problems would be really high for D.

I am sure there are all sorts of thorns involved but for your example a somewhat arbitrarily defined fallback hierarchy of types from most complex to most simple, with it matching the most 'simple' that it can.

On 4/11/18 10:26 AM, ixid wrote:
> Is it possible to infer a template's return type from what it's assigned to? If not is this a difficult or worthless feature to add?
> 
> OUT fun(IN, OUT)(IN value) {
>      return value.to!OUT;
> }
> 
> void main() {
>      float a = 5.0;
>      int b = fun(a);
> }

Sort of:

void fun(IN, OUT)(IN value, out OUT result)
{
   result = value.to!OUT;
}

Other than that, it doesn't work.

-Steve