[Haskell-cafe] Skolems!

[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 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)
> 
> 
> _______________________________________________
> Haskell-Cafe mailing list
> Haskell-Cafe at haskell.org
> http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe