#7862: Could not deduce (A) from the context (A, ...) -------------------------------------+------------------------------------- Reporter: alang9 | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.8.3 (Type checker) | Keywords: Resolution: | Architecture: x86_64 (amd64) Operating System: Linux | Difficulty: Unknown Type of failure: GHC | Blocked By: rejects valid program | Related Tickets: Test Case: | Blocking: | Differential Revisions: | -------------------------------------+------------------------------------- Comment (by goldfire): The problem in comment:5 is unrelated to the original bug report -- it is a !PolyKinds issue. If you turn on `-fprint-explicit-kinds`, you get {{{ Could not deduce (Category k2 (Morphisms (k -> k -> *) k2 * c)) arising from a use of ‘Control.Category.id’ from the context (Category k c, Category k1 (Morphisms (k -> k -> *) k1 * c)) bound by the instance declaration at Scratch.hs:30:10-64 or from (Objects (k -> k -> *) k2 c a) bound by the type signature for id :: (Objects (k -> k -> *) k2 c a) => Morphisms (k -> k -> *) k2 * c a a at Scratch.hs:33:5-6 In the expression: Control.Category.id In an equation for ‘id’: id = Control.Category.id In the instance declaration for ‘SmallCategory (k -> k -> *) c’ }}} and you can see a more sensible error message. What would be nice here is a note suggesting to turn on `-fprint-explicit- kinds`. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/7862#comment:7 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler