Memoization hack in Haskell, using unsafe IO operation.

memoize.hs

module Memoize (memoize) where

import Debug.Trace
import Data.Hashable
import System.IO.Unsafe

import qualified Data.HashTable.IO as H

type HashTable k v = H.CuckooHashTable k v

memoize :: (Eq k, Hashable k) => (k -> a) -> k -> a
memoize f = unsafePerformIO $ do
                cache <- H.new :: IO (HashTable k v)
                return $ \x -> unsafePerformIO $ do
                    tryV <- H.lookup cache x
                    case tryV of
                        Nothing -> do
                            traceM "New value"
                            let v = f x
                            H.insert cache x v
                            return v
                        Just v -> return v