[Haskell-cafe] Very evil

[Luke Palmer]

Maybe you would like the hetero-map package.  Its purpose is to do
precisely what you are doing, but in a typesafe way.

On Thu, Oct 15, 2009 at 3:10 PM, Andrew Coppin
<andrewcoppin at btinternet.com> 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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