Hi Stephen, I'm not sure I see the problem. You can do what you require with the function i supplied (minus the typo). This is in the module (where the Finite constructor is exposed)
finite :: b -> (a -> Finite s a -> b) -> Finite s a -> b finite b _ (Finite []) = b finite _ f (Finite (x:xs)) = f x (Finite xs) Then viewl can be defined anywhere, viewl :: Finite s a -> Either () (a, Finite s a) viewl = finite (Left ()) (Right . (,)) Why would you ever decrease the Peano numbers? It's just an upper bound, not an exact size. /J 2010/10/13 Stephen Tetley <stephen.tet...@gmail.com>: > Hi Jonas > > Thanks - I was meaning an equivalent to viewl on Data.Sequence, on plain lists: > > viewl :: [a] -> Either () (a,[a]) > viewl [] = Left () > viewl (x:xs) = Right (x,xs) > > > It was a trick question because I can't see how you can do it without > decrement on the Peano numbers. > > Best wishes > > Stephen >
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