like [(x, y) | x <- xs | y <- ys], and it's not clear how to define "zip" for a monad - but perhaps there is some extension of a monad where it makes sense?
Well, I question that the above notation makes sense (for lists). It is trying to be too clever. "standard" list comprehensions at least are consistent with mathematical notation for sets. (That is, they are putting the cart before the horse consistently.) But could you show the above code example to some non-ghc-aware person and expect her to guess the meaning correctly? I think not. And even if, the implied zip is dangerous because it does not complain about unequal lengths, and you cannot guess that either. If you write (x,y) <- zip xs ys instead then at least you know that you need to lookup the definition of zip. Best regards, J.W.