Differing levels of type-inference: Can D do this?

[Simen Kjaeraas]

On Sun, 29 Jul 2012 19:11:17 +0200, Chad J  
<chadjoan at __spam.is.bad__gmail.com> wrote:

>> So there. Now, you simply use auto a = constrain!isInputRange(
>> expression );. Is this what you wanted?
>
> That's pretty good.  It's still not as concise or easy to discover as  
> the language's natural syntax for type declarations, but it's the kind  
> of thing I'd use for my own purposes as a trick to get around language  
> limitations.

It's likely as good as it gets without changing the language. Of course,
if you use this a lot with the same predicate, you could alias it to
something shorter:

alias constrain!isInputRange InputRange;

auto a = InputRange(expression);

Note also that the implementation supports multiple predicates, hence the
supplied hasElementType. And this is where aliases really come in handy:

template hasElementType( T ) {
     template hasElementType( U ) {
         enum hasElementType = is( ElementType!U == T );
     }
}

alias constrain!(isInputRange, hasElementType!int) IntRange;

auto a = IntRange([1,2,3]);

-- 
Simen