Re: 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.