One more simple idea for simplifying constraints

[deadalnix via Digitalmars-d]

On Tuesday, 26 April 2016 at 03:17:09 UTC, Stefan Koch wrote:
> 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.

It is not semantically equivalent.