Thanks,
but you are assuming that foldl evaluates the intermediate result right away.
It must
My understanding of foldl is thus:
foldl (++) [] [[1],[2],[3],[4]]
[snip]
-> [] ++ [1] ++ [2] ++ [3] ++ [4]
Which brings me back to my original question...
Well, you've removed the parentheses that give you the information you want. foldl (++) [] [[1],[2],[3],[4]] -> foldl (++) ([] ++ [1]) [[2],[3],[4]] -> foldl (++) (([] ++ [1]) ++ [2]) [[3],[4]] -> foldl (++) ((([] ++ [1]) ++ [2]) ++ [3]) [[4]] -> foldl (++) (((([] ++ [1]) ++ [2]) ++ [3]) ++ [4]) [] -> (((([] ++ [1]) ++ [2]) ++ [3]) ++ [4]) now you have to ask what has to be evaluated in order to get the head of the result. The answer is that you have to evaluate ((([] ++ [1]) ++ [2]) ++ [3]) before you can find it, and to get that, you have to evaluate (([] ++ [1]) ++ [2]) and so on. In the other case, you can get the head of (a:...)++ (b ++ c) by evaluating only the first steps of the first (++). Does that help? Jón -- Jón Fairbairn Jon.Fairbairn@cl.cam.ac.uk 31 Chalmers Road jf@cl.cam.ac.uk Cambridge CB1 3SZ +44 1223 570179 (after 14:00 only, please!)