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

[Jonathan M Davis]

On Saturday, July 28, 2012 16:47:01 Chad J wrote:
> "range kind" is informal language.  Maybe I mean "template instances",
> but that would somewhat miss the point.
> 
> I don't know how to do this right now.  AFAIK, it's not doable.
> When I speak of ranges I refer specifically to the std.phobos ranges.
> There is no Range type right now, but there are the isInputRange,
> isOutputRange, isForwardRange, etc. templates that define what a Range
> is.  The problem is that I have no idea how to write something like this:
> 
> isInputRange!___ r2 = [1,2,3].some.complex.expression();
> 
> It doesn't make sense.  isInputRange!() isn't a type, so how do I
> constrain what type is returned from some arbitrary expression?

Well, if you want a check, then just use static assert.

auto r2 = [1,2,3].some.complex.expression();
static assert(isInputRange!(typeof(r2)));

The result isn't going to magically become something else just because you 
want it to, so all that makes sense is specifically checking that its type is 
what you want, and static assert will do that just fine.

This is completely different from template constraints where the constraint can 
be used to overload functions and generate results of different types depending 
on what's passed in. With the code above, it's far too late to change any 
types by the time r2 is created.

- Jonathan M Davis