[Haskell-cafe] Different behaviour with -XAllowAmbiguousTypes in 7.10.3b and 8.0.1

[Max Amanshauser]

Hi,

while porting a library to Haskell, which deals with persisting finite state automata to various stores, depending on the user's choice and the instances provided for types s(tate) e(vent) a(ction), I ran into different behaviour in ghc 7.10.3b and 8.0.1 related to ambiguity checks. This is a minimised and somewhat contrived example:

> -- testme.hs
> 
> {-# LANGUAGE MultiParamTypeClasses #-}
> {-# LANGUAGE FlexibleInstances #-}
> {-# LANGUAGE AllowAmbiguousTypes #-}
> 
> module TestMe where
> 
> data MyStore = MyStore
> 
> class FSMStore st s e a where
>     fsmRead :: st -> String -> s
> 
> instance (Num s) => FSMStore MyStore s e a where
>     fsmRead st i = 23
> 
> get :: (FSMStore st s e a) => st -> String -> s
> get st i = fsmRead st i

With GHC 8.0.1 I get:

> *TestMe> :l test.hs 
> [1 of 1] Compiling TestMe           ( test.hs, interpreted )
> 
> test.hs:19:12: error:
>     • Could not deduce (FSMStore st s e0 a0)
>         arising from a use of ‘fsmRead’
>       from the context: FSMStore st s e a
>         bound by the type signature for:
>                    get :: FSMStore st s e a => st -> String -> s
>         at test.hs:18:1-47
>       The type variables ‘e0’, ‘a0’ are ambiguous
>       Relevant bindings include
>         st :: st (bound at test.hs:19:5)
>         get :: st -> String -> s (bound at test.hs:19:1)
>       These potential instance exist:
>         instance Num s => FSMStore MyStore s e a
>           -- Defined at test.hs:12:10
>     • In the expression: fsmRead st i
>       In an equation for ‘get’: get st i = fsmRead st i
> Failed, modules loaded: none.

However, when I remove the type signature for get:

> Prelude> :l test.hs 
> [1 of 1] Compiling TestMe           ( test.hs, interpreted )
> Ok, modules loaded: TestMe.
> *TestMe> :t get
> get :: FSMStore st s e a => st -> String -> s
> *TestMe> get MyStore "asdf"
> 23

GHC inferred the exact same type I provided, but this time it compiles successfully.

When going back to GHC 7.10.3b, without type signature:

> GHCi, version 7.10.3: http://www.haskell.org/ghc/  :? for help
> :Prelude> :l test.hs 
> [1 of 1] Compiling TestMe           ( test.hs, interpreted )
> 
> test.hs:19:1:
>     Could not deduce (FSMStore st s e0 a0)
>     from the context (FSMStore st s e a)
>       bound by the inferred type for ‘get’:
>                  FSMStore st s e a => st -> String -> s
>       at test.hs:19:1-23
>     The type variables ‘e0’, ‘a0’ are ambiguous
>     When checking that ‘get’ has the inferred type
>       get :: forall st s e a. FSMStore st s e a => st -> String -> s
>     Probable cause: the inferred type is ambiguous
> Failed, modules loaded: none.

So my questions are:
*) What's with the different behaviour depending on whether the type sig is inferred or provided?
*) Is GHC 7.10.3b or 8.0.1 closer to the correct behaviour w.r.t. -XAllowAmbiguousTypes?

--
Regards,
Max Amanshauser.