• ixid (10/10) Apr 11 2018 Is it possible to infer a template's return type from what it's
• Adam D. Ruppe (12/15) Apr 11 2018 Not really. The function call needs to make sense by itself:
• ixid (5/20) Apr 11 2018 I am sure there are all sorts of thorns involved but for your
• Steven Schveighoffer (8/19) Apr 11 2018 Sort of:

ixid <nuaccount gmail.com> writes:

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);
}

Adam D. Ruppe <destructionator gmail.com> writes:

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.

ixid <nuaccount gmail.com> writes:

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.

Steven Schveighoffer <schveiguy yahoo.com> writes:

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