When I compile this with 7.8.3, it gives an error message saying that type
variables a and m are not in scope. If I add them to the forall, it tells
me I need FlexibleContexts. If I add that, then it gives me an error about
an ambiguous type variable. Clearly, something crashed ghc-4.04proto, but
there's no indication of what that was. If this test is still at all
relevant, it probably needs to be updated to target something more narrowly.
The program:
{-# LANGUAGE MultiParamTypeClasses, ExplicitForAll #-}
module ShouldFail where
-- !!! For-all with parens
-- This one crashed ghc-4.04proto; the parens after the for-all fooled it
class Monad m => StateMonad s m where
getState :: m s
setState0 :: forall b. (StateMonad (a,b) m => m a)
setState0 = getState >>= \ (l,_r) -> return l
David Feuer
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