{-# 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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