[Haskell-cafe] Memoisation + unsafePerformIO

[Tony Morris]

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Hello everyone,
I am trying to write a generalisation of memoisation for a couple of
backtracking algorithms that I wrote (which don't specify a memoisation
technique), by keeping a local destructive update using unsafePerformIO
and IORef - neither of which I have used before and I am struggling to
figure out. I am going from the document:
http://haskell.org/hawiki/GlobalMutableState

I am not sure if:
a) this is even possible
b) if a) holds, if this is a good thing

Here is some code to help demonstrate what I intend:

{-# OPTIONS_GHC -fglasgow-exts #-}
import Data.Ix

- -- implementations must guarantee that
- -- memo f k is equivalent to f k
- -- but may otherwise use local destructive updates
class Memo k v where
  memo :: (k -> v) -> k -> v

instance (Ix k) => Memo k v where
  memo f k = error("todo: array")

instance (Ord k) => Memo k v where
  memo f k = error("todo: binary tree")

- --
Tony Morris
http://tmorris.net/

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