In S. Thompson's book, problem 9.13 asks us to define 'init' in terms of foldr. I was baffled at first because I didn't see a natural way to do this. It would look something like init xs = foldr f initialValue xs where f would cons on each character except the rightmost. f <when passed rightmost char> b = [] f <when passed any other char a> b = a : b How does f "know" when it is passed the first character? initialValue has to signal this somehow. On #haskell, one person suggested doing it with some post-processing: init xs = snd $ foldr f (True,[]) xs where f _ (True,_) = (False,[]) f a (False,b) = (False,a:b) I had an idea. If the initial value is the entire list, then its length can function as the "signal" that we are dealing with the rightmost char. This requires no post-processing: init xs = foldr f xs xs where f a b | length b == length xs = [] | otherwise = a:b These seem contrived. I wonder if there is a more natural solution that Thompson had in mind. Any comments? -Mike