Prelude> let diag = concat . diags where diags ((x:xs):xss) = [x] : zipWith (:) xs (diags xss)
this has a different semantics on finite lists, so I should add a test case: *Main> diag [[1,2,3],[4,5,6],[7,8,9]] [1,2,4,3,5,7,6,8,9] Your version yields [1,2,4,3,5,7]. Actually, there are a number of implementations that implement the same behaviour as the original version, e.g., diag = concat . foldr diags [] where diags [] ys = ys diags (x:xs) ys = [x] : merge xs ys merge [] ys = ys merge xs@(_:_) [] = map (:[]) xs merge (x:xs) (y:ys) = (x:y) : merge xs ys I'd be interested if one can *derive* from the original version a simpler version using clever pointfree combinators. Cheers, Sebastian