One more simple idea for simplifying constraints

[Stefan Koch via Digitalmars-d]

On Monday, 25 April 2016 at 18:50:28 UTC, Andrei Alexandrescu 
wrote:
> Sometimes we use constraints to distinguish different approach 
> to implementation, e.g. find for a string is different than 
> find for forward ranges is different from find for input ranges 
> etc.
>
> One simple idea here (which Walter also advised a while ago) is 
> to push implementation details inside the, well, 
> implementation, e.g.:
>
> Type functionName(TemplateArgs)(Args)
> if (HighLevelConstraint)
> {
>   static if (LowLevelCapability1)
>   {
>     .. specialization 1 ...
>   }
>   else static if (LowLevelCapability1)
>   {
>     ... specialization 2 ...
>   }
>   else
>   {
>     .. conservative impl ...
>   }
> }
>
> That way the user/documentation only deals with the high level 
> constraint (the minimum necessary for the function to work). 
> Inside, the function decides the exact approach.
>
> It's an obvious idea now but many times in the past I didn't 
> pursue it.
>
>
> Andrei

While working on implementing template constraints for SDC.
I had a similar idea, basically a template constraint can be 
rewritten as a static if inside the template.


R Tmpl (U) (U u) if (is(U == int)) { pragma(msg, "U is an int"); }
R Tmpl (U) (U u) if (is(U == string)) { pragma(msg, "U is a 
string"); }

is equivalent to

R Templ (U u) {
   static if (is(U == int)) {
       pragma(msg, "U is an int");
   } else static if (is(U == string)) {
       pragma(msg, "U is a string");
   } else
       static assert(0, "Can only be instantiated with int or 
string");
}

Except that you just get one error message instead of 2 overloads 
displayed.