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{-# LANGUAGE DeriveDataTypeable, StandaloneDeriving #-}
-- | This shows how to use typeable to perform unsafe
-- casts. Basically its an example of what Safe Haskell
-- should disallow. SafeLang14 will do that.
module Main where
import SafeLang13_A
import Data.Typeable
data H = H String deriving (Typeable, Show)
data G = G Int deriving (Show)
instance Typeable G where
typeOf _ = typeOf (undefined :: H)
instance Typeable P where
typeOf _ = typeOf (undefined :: G)
{-
deriving instance Typeable G
deriving instance Typeable P
-}
main = do
let h = H "Hello World"
g = G 1
-- Just h' = (cast h) :: Maybe G
Just p' = (cast p) :: Maybe G
Just px = (cast $ incrG p') :: Maybe P
putStrLn $ show h
putStrLn $ show g
-- putStrLn $ show h'
putStrLn $ showP p
putStrLn $ show p'
putStrLn $ showP px
incrG :: G -> G
incrG (G n) = G $ n + 1
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