#11947: GHC mistakenly warns about type defaulting in the presence of -XTypeApplications -------------------------------------+------------------------------------- Reporter: RyanGlScott | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler (Type | Version: 8.0.1 checker) | Keywords: Resolution: | TypeApplications Operating System: Unknown/Multiple | Architecture: Type of failure: Incorrect | Unknown/Multiple warning at compile-time | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by RyanGlScott): Replying to [comment:3 goldfire]:
`theFloatDigits` isn't the thing that's ambiguous or not; it's `theFloatDigits`'s type, `forall a. RealFloat a => Int`, which certainly is ambiguous, mentioning a type variable only in a constraint and not the type head.
Sorry, I was confused. You're absolutely right in that `theFloatDigits`'s type signature is ambiguous. That is apparent if you try to typecheck something similar without defaulting rules like: {{{#!hs theIsSigned :: forall a. Bits a => Bool theIsSigned = isSigned (undefined @_ @a) }}} This fails to compile without `-XAllowAmbiguousTypes` enabled, which makes sense. `theFloatDigits`, on the other hand, compiles both with and without `-XAllowAmbiguousTypes`! This adds another wrinkle to the bug—should it require `-XAllowAmbiguousTypes`, or should it type defaulting make certain otherwise-ambiguous type signatures permissible? I'm not sure what the answer is, but I do believe that we should at least have some way to write `theFloatDigits` without needing to disable `-Wtype-defaults` altogether. Would it make sense to disable type defaulting in ambiguity checks when `-XAllowAmbiguousTypes` is enabled? After all, the warning message already suggests turning it on. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11947#comment:4 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler