#9747: Odd failure to deduce a constraint -------------------------------------+------------------------------------- Reporter: acowley | Owner: Type: bug | Status: closed Priority: normal | Milestone: Component: Compiler | Version: 7.8.3 Resolution: fixed | Keywords: TypeFamilies Operating System: | ConstraintKinds Unknown/Multiple | Architecture: Unknown/Multiple Type of failure: | Difficulty: Unknown None/Unknown | Blocked By: Test Case: | Related Tickets: Blocking: | Differential Revisions: | -------------------------------------+------------------------------------- Changes (by simonpj): * status: infoneeded => closed * resolution: => fixed Comment: It's a bug in GHC 7.8.3, fixed in upcoming 7.8.4 and HEAD, and not present in 7.8.2. Both say {{{ T9747.hs:35:8: Type family ‘ConFun’ should have 2 arguments, but has been given 1 In the type signature for ‘bar’: bar :: HListAll (ConFun l) ts => Proxy l -> HList ts -> () }}} And that's right! This works though {{{ type family ConFun (t :: *) :: * -> Constraint data Tag type instance ConFun Tag = Group class (Show a, Eq a, Ord a) => Group a bar :: HListAll (ConFun l) ts => Proxy l -> HList ts -> () bar _ _ = () baz :: (ConFun l a, ConFun l b) => Proxy l -> HList [a,b] -> () baz = bar }}} I've eta-reduced `ConFun`, but then I had to give a fresh name to the RHS via the class decl for `Group`. I'll add the working version as a regression test. Simon -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9747#comment:3 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler