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

20 Jul 2016


      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.