#9569: Tuple constraints don't work right -------------------------------------+------------------------------------- Reporter: simonpj | Owner: Type: bug | Status: closed Priority: normal | Milestone: Component: Compiler | Version: 7.8.2 Resolution: fixed | Keywords: Operating System: | Architecture: Unknown/Multiple Unknown/Multiple | Difficulty: Unknown Type of failure: | Blocked By: None/Unknown | Related Tickets: Test Case: | polykinds/T9569, | typecheck/should_compile/T9569a | Blocking: | Differential Revisions: | -------------------------------------+------------------------------------- Comment (by basvandijk): I think I was confused. The problem is not in `monad-control` but in `lifted-base` (which depends on `monad-control`). If I remove the `{-# LANGUAGE ImpredicativeTypes #-}` from `Control.Exception.Lifted` I get the following error on [https://github.com/basvandijk/lifted- base/blob/master/Control/Exception/Lifted.hs#L268 mask]: {{{ Control/Exception/Lifted.hs:268:8: Cannot instantiate unification variable ‘a2’ with a type involving foralls: (forall a. m a -> m a) -> m b Perhaps you want ImpredicativeTypes In the expression: liftBaseOp E.mask . liftRestore In an equation for ‘mask’: mask = liftBaseOp E.mask . liftRestore }}} Is this easy solvable (preferably without changing the API)? -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9569#comment:13 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler