On Wednesday 01 August 2007 17:44, Thomas Conway wrote:
This sounds like a common problem type. Is there a well known solution to this sort of problem?
Have you looked into Tying the Knot? http://www.haskell.org/haskellwiki/Tying_the_Knot A simple example: module Knot where import Data.Char import Data.Maybe type Input = String type Output = [(Char, Int)] type Resolver = (Char -> Int) resolvingError c = error ("Couldn't resolve: " ++ [c]) parseInput :: Resolver -> Input -> Output parseInput _ [] = [] parseInput resolve (c:cs) | isUpper c = ((c, fromEnum c) : parseInput f cs) | otherwise = ((c, resolve c) : parseInput f cs) makeResolver :: Output -> Resolver makeResolver o c = fromMaybe (resolvingError c) (lookup (toUpper c) o) foo :: Input -> Output foo i = let o = parseInput (makeResolver o) i in o testGood = foo "CaBcbA" testBad = foo "CaBcb"