#12975: Suggested type signature for a pattern synonym causes program to fail to type check -------------------------------------+------------------------------------- Reporter: ocharles | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler (Type | Version: 8.0.1 checker) | Keywords: Resolution: | PatternSynonyms Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by simonpj): The reported type is sort of right: {{{ Cast :: forall b. Typeable b => forall a. Typeable a => b -> T }}} That is, you can match a value of any type `b` provided it is typeable. So {{{ f (Cast x) = rhs }}} means pretty much {{{ f (MkT y) = case cast y of Just x -> rhs }}} Inside `rhs` we have access to the `Typeable a` constraint bound by `MkT`; that's what the `Typeable a` constraint in the inferred type is saying. But this `Typeable a` provided constraint is no use to `rhs` because we have no values of type `a`. So Matthew's type is less confusing {{{ pattern Cast :: Typeable b => b -> T }}} Offering useless constraints is bad enough; but worse, as you report, it's rejected as ambiguous (for the same reason) if you specify it in a user signature. That is indeed confusing. What I can't see is a robust and convenient way to eliminate the unnecessary constraints and existential variables. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12975#comment:5 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler