Metaprogramming in D : Some Real-world Examples

[Max Samukha]

On Wed, 11 Nov 2009 10:30:24 -0600, Andrei Alexandrescu
<SeeWebsiteForEmail at erdani.org> wrote:

>
>Then different isXxxRange are used by higher-order ranges in defining 
>refined interfaces depending on the interfaces offered by their inputs. 
>I fail to see how that's terrible.

It is not terrible. It just doesn't give any clue about what exactly
is wrong with the entity for which interface validation has failed. I
tend to put the code that defines the interface in a separate template
to be able to find out the exact error.

And because neither _traits(compiles) nor is(typeof) can be currently
used to test template instantiations, the code goes to a mixed-in
string and the whole affair gets messy. This is the result:

private enum inputRangeConcept = q{{

        R r;             // can define a range object
        if (r.empty) {}  // can test for empty
        r.popFront;          // can invoke next
        auto h = r.front; // can get the front of the range
     }};

template InputRangeConcept(R)
{
   mixin ("void _()" ~ inputRangeConcept);
}

template isInputRange(R)
{
  enum isInputRange = is(typeof(mixin(inputRangeConcept)));
}

So, for example:

alias InputRangeConcept!(int) check;

will yield

test.d(20): Error: no property 'empty' for type 'int'
test.d(21): Error: no property 'popFront' for type 'int'
test.d(22): Error: no property 'front' for type 'int'

which gives exact information about what is wrong with int candidate
for input range position (line numbers are incorrect, though).