#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 simonpj): Hmm. Here is what is happening. * `tmp` is not generalise because of the monomorphism restriction. So we get `tmp :: alpha`, for some as-yet-unknown type `alpha`; plus the constraint `Num alpha`. * `MonoLocalBinds` prevents generalisation of a binding if it mentions anything that has an open type; that is, a type with free type variables. `tmp` does, so `picker` is not generalised. So it gets type `picker :: gamma -> gamma -> gamma`. * Now we apply picker to both `[Char]` (from `picker "x" "y"`) and `delta` (where we also need `Num delta`) from `picker 10 11`. So we say `gamma := delta := [Char]` and get stuck on `Num [Char]`. That's the diagnosis. The error message is unhelpful -- but it's not obvious to me how to improve it. What error message would you like? -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15931#comment:1 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler