[Haskell-cafe] Skolems!

[evan@evan-borden.com]

Thanks Richard, will do.

On Mon, Mar 2, 2015 at 1:12 PM, Richard Eisenberg <eir at cis.upenn.edu> wrote:

> 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 at evan-borden.com" <
> evan at 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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>
>
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