Re: [Haskell-beginners] Graham Scan exercise from Chapter 3 RWH -Spoilers. Don't read if you want to do this exercise.

On 3 September 2010 11:57, Jürgen Doser wrote:

It is tempting to use some kind of map or foldr for this. Unfortunately, there isn't a really nice way. Such a "sliding window" map is occasionally useful, but there is no pre-defined function for it in the libraries.

Though not in Data.List, paramorphism is the standard 'sliding window' fold. -- paramorphism (generalizes catamorphism i.e. foldr) -- para :: (a -> ([a], b) -> b) -> b -> [a] -> b para phi b = step where step [] = b step (x:xs) = phi x (xs, step xs) With a paramorphism the 'lookahead' is left-to-right, but the list itself is processed from the right (as per a right-fold). This can sometimes make it a little tricky to use.