#9624: "Unlikely constraint" recognition -------------------------------------+------------------------------------- Reporter: dfeuer | Owner: Type: feature request | Status: new Priority: normal | Milestone: Component: Compiler (Type | Version: 7.8.3 checker) | Operating System: Keywords: | Unknown/Multiple Architecture: Unknown/Multiple | Type of failure: Difficulty: Unknown | None/Unknown Blocked By: | Test Case: Related Tickets: | Blocking: | Differential Revisions: -------------------------------------+------------------------------------- Some constraints or combinations are relatively unlikely to appear in correct code, or at least correct code written by beginners. A few tiny examples: {{{#!hs (Integral a, Fractional a) => -- Possible, but rare, in correct code. (Integral a, RealFloat a) => -- Rather less possible in correct code. (Num a, Foldable a) => -- Possible, but unlikely for a beginner. (Fractional [a]) => (Show (A -> B)) => }}} The current error messages when constraints like these are not satisfied does nothing to suggest that they are unlikely to be correct or to explain to a beginner what kinds of mistakes can lead to errors like this. I imagine some people who teach classes using Haskell will have an idea of which specific things are worth singling out for a special explanation. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9624 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler