Different behaviour with -XAllowAmbiguousTypes in 7.10.3b and 8.0.1
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.
Hi Max, I think 7.10.3 is correct to reject both programs. The `get` function with a type signature is ambiguous, even when AllowAmbiguousTypes is enabled, because there is no reason for GHC to pick `e` and `a` when instantiating the type variables in the call to `fsmRead`. In general, if a function's type can be inferred, it should be possible to give it a signature with that type. 8.0.1 apparently doesn't respect this property, which is a bug. AllowAmbiguousTypes should make it possible to write a function with the same type signature as `get`, e.g. by using TypeApplications to fix the variables: get :: forall st s e a. (FSMStore st s e a) => st -> String -> s get st i = fsmRead @st @s @e @a st i Of course, this merely defers the ambiguity to the call sites of `get`. Hope this helps, Adam On 20/07/16 16:52, Max Amanshauser wrote:
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.
participants (2)
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Adam Gundry -
Max Amanshauser