#15931: MonoLocalBinds + MonomorphismRestriction prevents generalization for a top level definition -------------------------------------+------------------------------------- Reporter: theindigamer | Owner: (none) Type: bug | Status: new Priority: low | Milestone: ⊥ Component: Compiler | Version: 8.6.2 Resolution: | Keywords: | MonoLocalBinds, | MonomorphismRestriction Operating System: Unknown/Multiple | Architecture: Type of failure: Incorrect | Unknown/Multiple error/warning at compile-time | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by theindigamer): ==== Proposal 1 (somewhat radical) In my experience, such error messages (the ones which are inaccurate, in contrast to most other messages) very often involve the MonomorphismRestriction in one way or another, as well as some function in the middle not having an explicit signature. Often the problem will go away once you add that missing signature. So, if you get an error with MonomorphismRestriction on, and the error involves functions which do not have explicit signatures ("involves" would need to be more precisely defined...) - try turning it off and see if the error goes away. If it does, inform the user the "hey, MonomorphismRestriction is causing a problem, so please annotate foo with the type X" that I inferred with NoMonomorphismRestriction. I don't know how practical this is or whether it could lead to undesirable consequences. ==== Proposal 2 In this particular case at least, a better error message would be with the substituted types - {{{ Inferred picker :: [Char] -> [Char] -> [Char] due to 7 | picker "x" "y" and also picker :: Num a => a -> a -> a due to 8 | picker 10 11 which conflict. Looks like picker doesn't have an explicit type signature - try providing one to fix the issue, or get a more informative error message. }}} Of course, the exact wording itself is not important, but now it is much clearer what the source of the problem is. I'm not sure what the appropriate level of generality is here (to give this kind of error message). -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15931#comment:2 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler