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{-# LANGUAGE GADTs #-}
module Main where
import Data.IORef
data T a where
Li:: Int -> T Int
Lb:: Bool -> T Bool
La:: a -> T a
writeInt:: T a -> IORef a -> IO ()
writeInt v ref = case v of
~(Li x) -> writeIORef ref (1::Int)
readBool:: T a -> IORef a -> IO ()
readBool v ref = case v of
~(Lb x) ->
readIORef ref >>= (print . not)
tt::T a -> IO ()
tt v = case v of
~(Li x) -> print "OK"
main = do
tt (La undefined)
ref <- newIORef undefined
writeInt (La undefined) ref
readBool (La undefined) ref
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