blob: fd4a8fcda6605dc41f21085c9c81cc2ca0376c67 (
plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
|
{-# LANGUAGE TypeFamilies, ScopedTypeVariables,
GeneralizedNewtypeDeriving #-}
module T7148a where
import Control.Monad.ST
data Proxy a = Proxy
type family Result a b
class Convert a where
coerce :: Proxy b -> a -> Result a b
newtype SAFE a = SAFE a
type instance Result (SAFE a) b = a
instance Convert (SAFE a) where
coerce _ (SAFE a) = a
newtype IS_NO_LONGER a = IS_NO_LONGER a deriving Convert
type instance Result (IS_NO_LONGER a) b = b
--inferred type is
unsafeCoerce :: forall a b. a -> b
unsafeCoerce = coerce (Proxy :: Proxy b) . IS_NO_LONGER . SAFE
--use it safely
id' :: a -> a
id' = unsafeCoerce
--segfault (with high probability)
crash :: segfault
crash = unsafeCoerce . tail . tail . tail . unsafeCoerce $ True
--time for side effects
unsafePerformIO :: IO a -> a
unsafePerformIO x = runST $ unsafeCoerce x
|
