#9973: Erroneous Redundant constraint warning -------------------------------------+------------------------------------- Reporter: alanz | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.11 Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: None/Unknown Unknown/Multiple | Blocked By: Test Case: | Related Tickets: Blocking: | Differential Revisions: | -------------------------------------+------------------------------------- Using version 4425ab99d6410839fa7567950b0a4696b0a3d70f with D538 applied, If the attached source file is loaded as is into ghci it generates the following warning for line 5, but otherwise loads fine {{{ /home/alanz/mysrc/github/alanz/HaRe/Bug2.hs:5:18: Warning: Redundant constraint: SYB.Data t In the type signature for: duplicateDecl :: SYB.Data t => [GHC.LHsBind GHC.Name] -> t -> GHC.Name -> GHC.Name -> IO [GHC.LHsBind GHC.Name] }}} If the constraint is commented out, it results in an error {{{ /home/alanz/mysrc/github/alanz/HaRe/Bug2.hs:26:16: No instance for (SYB.Data t) arising from a use of ‘definingSigsNames’ Possible fix: add (SYB.Data t) to the context of the type signature for: duplicateDecl :: [GHC.LHsBind GHC.Name] -> t -> GHC.Name -> GHC.Name -> IO [GHC.LHsBind GHC.Name] In the expression: definingSigsNames [n] sigs In an equation for ‘typeSig’: typeSig = definingSigsNames [n] sigs In an equation for ‘duplicateDecl’: duplicateDecl decls sigs n newFunName = do { let sspan = ...; newSpan <- case typeSig of { [] -> return sspan _ -> ... }; let rowOffset = ...; .... } where typeSig = definingSigsNames [n] sigs Failed, modules loaded: none. }}} This behaviour goes away if the following lines are commented out {{{#!hs let rowOffset = case typeSig of [] -> 2 _ -> 1 }}} -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9973 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler