#8080: Typechecking error: "Could not deduce (m ~ m1)..." -------------------------------------+------------------------------------- Reporter: abacabadabacaba | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler (Type | Version: 7.6.3 checker) | Operating System: Unknown/Multiple Keywords: | Type of failure: GHC rejects Architecture: Unknown/Multiple | valid program Difficulty: Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | -------------------------------------+------------------------------------- This program: {{{ import Control.Monad f x = let g y = let h z = liftM not x in h 0 in g 0 }}} is expected to compile. Instead, it produces an error: {{{ test.hs:5:21: Could not deduce (m ~ m1) from the context (Monad m) bound by the inferred type of f :: Monad m => m Bool -> m Bool at test.hs:(3,1)-(7,8) or from (m Bool ~ m1 Bool, Monad m1) bound by the inferred type of h :: (m Bool ~ m1 Bool, Monad m1) => t1 -> m1 Bool at test.hs:5:5-21 `m' is a rigid type variable bound by the inferred type of f :: Monad m => m Bool -> m Bool at test.hs:3:1 `m1' is a rigid type variable bound by the inferred type of h :: (m Bool ~ m1 Bool, Monad m1) => t1 -> m1 Bool at test.hs:5:5 Expected type: m1 Bool Actual type: m Bool In the second argument of `liftM', namely `x' In the expression: liftM not x In an equation for `h': h z = liftM not x }}} -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/8080 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler