[Haskell-cafe] Backpatching

[Daniel McAllansmith]

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"