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{-# OPTIONS_GHC -fno-warn-redundant-constraints #-}
{-# LANGUAGE RankNTypes, ExistentialQuantification #-}
-- An interesting interaction of universals and existentials, prompted by
-- http://www.haskell.org/pipermail/haskell-cafe/2004-October/007160.html
--
-- Note the nested pattern-match in runProg; tc183 checks the
-- non-nested version
-- 3 Sept 2010: with the new typechecker, this one succeeds
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 (Bar run op)) = run (prog op)
-- This nested match "ought" to work; because
-- runProg (Foo b) = case b of
-- Bar run op -> run (prog op)
-- does work. But the interactions with GADTs and
-- desugaring defeated me, and I removed (in GHC 6.4) the ability
-- to instantiate functions on the left
prog :: (MonadTrans t, Monad (t IO)) => a -> t IO ()
prog x = error "urk"
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