#10202: Weak inference when using rank-2 types and type families. -------------------------------------+------------------------------------- Reporter: diatchki | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.10.1 (Type checker) | Operating System: Unknown/Multiple Keywords: | Type of failure: GHC rejects Architecture: | valid program Unknown/Multiple | Blocked By: Test Case: | Related Tickets: Blocking: | Differential Revisions: | -------------------------------------+------------------------------------- Type inference does work as expected, when a rank-2 type has type-family constraint. Consider the following program: {{{ {-# LANGUAGE TypeFamilies #-} {-# LANGUAGE Rank2Types #-} type family F a f :: (forall s. (F s ~ Int) => s -> b) -> b f _ = undefined k :: s -> Char k = undefined example :: Bool example = const True (f k) }}} It is rejected with the following error: {{{ Couldn't match type ‘b0’ with ‘Char’ ‘b0’ is untouchable inside the constraints (F s ~ Int) bound by a type expected by the context: (F s ~ Int) => s -> b0 at bug.hs:13:23-25 Expected type: s -> b0 Actual type: s -> Char In the first argument of ‘f’, namely ‘k’ In the second argument of ‘const’, namely ‘(f k)’ }}} This is unexpected because the result of `f` should be the same as the result of its parameter, and we know the exact type of the parameter, namely `Char`. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/10202 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler