Why can't you get the length of (,,)?

Question

length (1,2) 

returns a result, but

length (1, 2, 3)

gives an error. Why is that?

Reading the types of (,) and (,,) did not help.

Edit: I'm thinking of the length from Data.Foldable length :: Foldable t => t a -> Int

Answer 1

The Functor, Traversable, and most especially Foldable instances for pairs are somewhat controversial. There are circumstances where they allow code to compile and do something that was not intended. I proposed adding the missing functor instances for triples, etc. and, as the ensuing threads show, that really touched a nerve. So why doesn't (,,) have a Foldable instance? Because the community hasn't reached a consensus that it should.