LEM.hs

{-# LANGUAGE DeriveDataTypeable #-}

module LEM where

import Control.Exception
import Data.Typeable
import System.IO.Unsafe

data Void

absurd :: Void -> a
absurd a = a `seq` error "Value of type void."

type Lem a = Either a (a -> Void)
newtype Wrap a = Wrap { unwrap :: a } deriving (Typeable, Show)

instance (Show a, Typeable a) => Exception (Wrap a)

cut :: (Show a, Typeable a) => (Lem a -> c) -> c
cut f = unsafePerformIO $ catch lie recover
  where lie     = return $! f $ Right $ throw . Wrap
        recover = return . f . Left . unwrap