Recursive vs. iterative constraints

[Andrei Alexandrescu via Digitalmars-d]

So the constraint on chain() is:

Ranges.length > 0 &&
allSatisfy!(isInputRange, staticMap!(Unqual, Ranges)) &&
!is(CommonType!(staticMap!(ElementType, staticMap!(Unqual, Ranges))) == 
void)

Noice. Now, an alternative is to express it as a recursive constraint:

(Ranges.length == 1 && isInputRange!(Unqual!(Ranges[0])))
   ||
   (Ranges.length == 2 &&
     isInputRange!(Unqual!(Ranges[0])) &&
     isInputRange!(Unqual!(Ranges[1])) &&
     !is(CommonType!(ElementType!(Ranges[0]), ElementType!(Ranges[1])) 
== void))
   || is(typeof(chain(rs[0 .. $ / 2], chain(rs[$ / 2 .. $]))))

In the latter case there's no need for additional helpers but the 
constraint is a bit more bulky.

Pros? Cons? Preferences?


Andrei