It looks like you may want http://www.haskell.org/ghc/docs/latest/html/libraries/base/Data-Dynamic.html . It does more or less what you're talking about behind the scenes, but probably uses the magical Any type to not encounter issues with functions, and gives you a Nothing if you attempt to coerce it to the wrong type. On Thu, Oct 15, 2009 at 5:10 PM, Andrew Coppin wrote:

{-# LANGUAGE EmptyDataDecls #-}

module Main (main) where

import Unsafe.Coerce

data Anything

newtype Key x = Key Int deriving Eq

type Dict = [(Key Anything, Anything)]

put :: Key x -> x -> Dict -> Dict put k' v' = raw (unsafeCoerce k') (unsafeCoerce v')  where   raw k0 v0 [] = [(k0,v0)]   raw k0 v0 ((k,v):zs)     | k == k0   = (k0 ,v0) :           zs     | otherwise = (k  ,v ) : raw k0 v0 zs

get :: Key x -> Dict -> Maybe x get k' zs = unsafeCoerce (raw (unsafeCoerce k') zs)  where   raw k0 [] = Nothing   raw k0 ((k,v):zs)     | k == k0   = Just v     | otherwise = raw k0 zs

main = do  let k1 = Key 1 :: Key Int  let k2 = Key 2 :: Key Double  let k3 = Key 3 :: Key String  let k4 = Key 4 :: Key Bool

 let d0 = []  let d1 = put k1  123  d0  let d2 = put k2  123  d1  let d3 = put k3 "123" d2  let d4 = put k4 True  d3

 print (get k1 d4)  print (get k2 d4)  print (get k3 d4)  print (get k4 d4)

Unsafe coerce, anyone?

This particular example appears to run without incident, but the GHC docs suggest that this is very... well, unsafe. (In particular, the docs claim this will fall over on function types.)

I presume there's some less-evil way of doing this?

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