#9110: MonomorphismRestriction too eager -------------------------------------+------------------------------------ Reporter: goldfire | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.8.2 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Unknown/Multiple Type of failure: None/Unknown | Difficulty: Unknown Test Case: | Blocked By: Blocking: | Related Tickets: -------------------------------------+------------------------------------ Comment (by simonpj): Here is what is happening: * `baz` gets the type `forall b. b -> alpha`, where `alpha` is a unification variable, plus a `Num alpha` constraint. The MR prevents us generalising the `alpha`. * Because `baz` is bitten by the MR, it is not considered "closed"; see [http://www.haskell.org/ghc/docs/7.8.2/html/users_guide/other-type- extensions.html#monomorphism 7.13.9.3 in the user manual]. Actually the user manual is badly framed here. It should say "A binding is considered closed if (a) all its free variables are imported or closed, and (b) it is unaffected by the monomorphism restriction." * Because `baz` is not closed, and `bar` mentions `baz`, `bar` is not closed. Since `LocalMonoBinds` is force, `bar` is not generalised. * Since `bar` is not genrealised, `foo` really is not as polymorphic as its type signature claims. I'll fix the user manual, but otherwise things are working fine Simon -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9110#comment:2 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler