SE-0358: Primary Associated Types in the Standard Library

[ksluder (Kyle Sluder)]

@xwu

Wouldn’t the element type have to be (Left, Right), and thus give good reason for the use of a single primary associated type—i.e., PairwiseIterator<(Left, Right)>? This is why concrete examples are key here.

Just because the Element has to be a tuple doesn’t mean that the tuple should be the protocol’s single associated type. Why force programmers to type the extra parentheses? In fact, without additional constraints imposed by the protocol requirements, a protocol can’t actually enforce that its single associated type be a 2-tuple. That makes two good reasons for using two associated times, IMO.