#9019: Ambiguity checker overeager on "ambiguous" kind variables -------------------------------------+------------------------------------ Reporter: goldfire | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.8.2 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Unknown/Multiple Type of failure: None/Unknown | Difficulty: Unknown Test Case: | Blocked By: Blocking: | Related Tickets: -------------------------------------+------------------------------------ Comment (by simonpj): Sure it's ambiguous! Try {{{ lsoki :: Apply LeftSym0 arg ~ Left arg => Proxy arg -> LeftSym0 l lsoki = LSOKI }}} and you get the exact same error message. "Ambiguous" does not mean "can never under any circumstances be called". Here's a simpler example: {{{ class C (x :: Maybe a) data LeftSym0 where LSOKI :: C Nothing => LeftSym0 }}} This gives {{{ T9019.hs:9:3: Could not deduce (C k0 ('Nothing k0)) arising from the ambiguity check for the type of the constructor ‘LSOKI’ from the context (C k ('Nothing k)) bound by the type of the constructor ‘LSOKI’: C k ('Nothing k) => LeftSym0 at T9019.hs:9:3 The type variable ‘k0’ is ambiguous In the ambiguity check for: forall (k :: BOX). C k ('Nothing k) => LeftSym0 }}} And indeed, there is nothing to resolve that ambiguous 'k', just as there wasn't in the previous example. So this looks right to me. Simon -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9019#comment:1 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler