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{-# OPTIONS_GHC -fno-warn-redundant-constraints #-}
{-# LANGUAGE ExistentialQuantification, RankNTypes #-}
-- An interesting interaction of universals and existentials, prompted by
-- http://www.haskell.org/pipermail/haskell-cafe/2004-October/007160.html
--
-- Note the non-nested pattern-match in runProg; tcfail126 checks the
-- nested pattern match
module Foo where
import Control.Monad.Trans
data Bar m
= forall t. (MonadTrans t, Monad (t m))
=> Bar (t m () -> m ()) (t m Int)
data Foo = Foo (forall m. Monad m => Bar m)
runProg :: Foo -> IO ()
runProg (Foo b) = case b of
Bar run op -> run (prog op)
-- You can't say runProg (Foo (Bar run op));
-- see tcfail126
prog :: (MonadTrans t, Monad (t IO)) => a -> t IO ()
prog x = error "urk"
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