Thanks Richard, will do.
On Mon, Mar 2, 2015 at 1:12 PM, Richard Eisenberg
This is admittedly a dark corner of the inference algorithm, but perhaps these examples can shed some light. Post a bug report!
Thanks for creating nice, small examples.
Richard
PS: I offer no strong guarantees that you've found bugs here... but in any case, posting a bug report gets more serious attention than a Haskell-cafe post.
On Feb 27, 2015, at 1:23 PM, "evan@evan-borden.com" < evan@evanrutledgeborden.dreamhosters.com> wrote:
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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