The weirdest part of this to me is the very last line --
This typechecks:
_ -> "bar" :: String
But this complains about a (rigid, skolem) type (and "n" really is just a string):
_ -> fst ("bar" :: String, n :: String)
Tom
El Feb 27, 2015, a las 13:23, "evan@evan-borden.com"
An extension of the message Tom sent a little while back, we've discovered a more in depth example of this possible GHC bug. It is exacerbated by GADTs, but can be fixed with NoMonoLocalBinds. Without GADTs and just leveraging ExistentialQuanitification it works fine. We've included a pretty exhaustive set of examples.
{-# LANGUAGE ExistentialQuantification, GADTs #-}
{- removing MonoLocalBinds fixes all of these errors {-# LANGUAGE ExistentialQuantification, GADTs, NoMonoLocalBinds #-} -}
module PossibleGHCBug where
data SumType = SumFoo | SumBar
class SomeClass a where someType :: a -> SumType
data SomeExistential = forall a. SomeClass a => SomeExistential a
noError :: String -> [SomeExistential] -> String noError n st = n ++ concatMap cname st where cname (SomeExistential p) = d p
d p = c $ someType p
c p = case p of SumFoo -> "foo" _ -> "asdf"
noError2 :: String -> [SomeExistential] -> String noError2 n st = n ++ concatMap cname st where cname (SomeExistential p) = d p
d p = c $ someType p
c :: SumType -> String c p = case p of SumFoo -> "foo" _ -> "asdf" ++ n
noError3 :: String -> [SomeExistential] -> String noError3 n st = n ++ concatMap cname st where cname (SomeExistential p) = d p
d :: SomeClass a => a -> String d p = c $ someType p
c p = case p of SumFoo -> "foo" _ -> "asdf" ++ n
partialTypedError :: String -> [SomeExistential] -> String partialTypedError n st = n ++ concatMap cname st where cname :: SomeExistential -> String cname (SomeExistential p) = d p
d p = c $ someType p
c p = case p of SumFoo -> "foo" _ -> "asdf" ++ n
fullError :: String -> [SomeExistential] -> String fullError n st = n ++ concatMap cname st where cname (SomeExistential p) = d p
d p = c $ someType p
c p = case p of SumFoo -> "foo" _ -> "asdf" ++ n
justNError :: String -> [SomeExistential] -> String justNError n st = n ++ concatMap cname st where cname (SomeExistential p) = d p
d p = c $ someType p
c p = case p of SumFoo -> "foo" _ -> n
ignoreNError :: String -> [SomeExistential] -> String ignoreNError n st = n ++ concatMap cname st where cname (SomeExistential p) = d p
d p = c $ someType p
c p = case p of SumFoo -> "foo" _ -> fst ("foo", n)
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