#11282: Error warns about non-injectivity of injective type family -------------------------------------+------------------------------------- Reporter: goldfire | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.11 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Old description:
When I say
{{{ {-# LANGUAGE TypeFamilies, RankNTypes #-}
module Bug where
import GHC.Exts
type family F a = r | r -> a type family G a :: Constraint
meth :: (G a => F a) -> () meth = undefined }}}
I get
{{{ Bug.hs:10:9: error: • Could not deduce: F a ~ F a0 from the context: G a0 bound by the type signature for: meth :: G a0 => F a0 at Bug.hs:10:9-26 NB: ‘F’ is a type function, and may not be injective The type variable ‘a0’ is ambiguous • In the ambiguity check for ‘meth’ To defer the ambiguity check to use sites, enable AllowAmbiguousTypes In the type signature: meth :: (G a => F a) -> () }}}
The presence of `G a` is critical for exhibiting this bug. If I replace `G a` with `Show a` the problem disappears.
This one was actually encountered in an attempt to do Useful Work, not just idle typecheckery.
New description: When I say {{{ {-# LANGUAGE TypeFamilies, RankNTypes #-} module Bug where import GHC.Exts type family F a = r | r -> a type family G a :: Constraint meth :: (G a => F a) -> () meth = undefined }}} I get {{{ Bug.hs:10:9: error: • Could not deduce: F a ~ F a0 from the context: G a0 bound by the type signature for: meth :: G a0 => F a0 at Bug.hs:10:9-26 NB: ‘F’ is a type function, and may not be injective The type variable ‘a0’ is ambiguous • In the ambiguity check for ‘meth’ To defer the ambiguity check to use sites, enable AllowAmbiguousTypes In the type signature: meth :: (G a => F a) -> () }}} The presence of `G a` is critical for exhibiting this bug. If I replace `G a` with `Show a` the problem disappears. This one was actually encountered in an attempt to do Useful Work, not just idle typecheckery. EDIT: Actually, I understand why this program should be rejected (comment:1), but the error message is surely misleading and should be fixed. -- Comment (by goldfire): I had figured that out (in rather less detail) by looking at the case without the injective type family. But the original error message is still warning me about a type family that isn't injective! So I think this boils down to adding a bit more logic to !TcErrors to print out an error about untouchable variables here instead of non-injective type families. I've modified the title and description to make this more clear. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11282#comment:3 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler