#9110: MonomorphismRestriction too eager ------------------------------------+------------------------------------- Reporter: goldfire | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.8.2 Keywords: | Operating System: Unknown/Multiple Architecture: Unknown/Multiple | Type of failure: None/Unknown Difficulty: Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | ------------------------------------+------------------------------------- This strange example came up on IRC: {{{ {-# LANGUAGE MonoLocalBinds, MonomorphismRestriction #-} foo :: a foo = bar bar = undefined where _ = baz undefined baz = const [1] }}} yields {{{ /Users/rae/temp/Bug.hs:4:7: Couldn't match expected type ‘a’ with actual type ‘t0’ because type variable ‘a’ would escape its scope This (rigid, skolem) type variable is bound by the type signature for foo :: a at /Users/rae/temp/Bug.hs:3:8 Relevant bindings include foo :: a (bound at /Users/rae/temp/Bug.hs:4:1) In the expression: bar In an equation for ‘foo’: foo = bar }}} Setting either `NoMonoLocalBinds` ''or'' `NoMonomorphismRestriction` fixes the problem. After some staring at this, I didn't get to the bottom of it. But I did learn some things: * If we remove the definition for `foo`, the module compiles. * With the monomorphism restriction (MR), `bar` gets the type `GHC.Exts.Any`. * Without the MR, `bar` gets the type `forall t.t`. * The MR bites `baz`, as it should. According to the rules in [http://www.haskell.org/onlinereport/decls.html#sect4.5.5 the Haskell Report], it seems that the MR should ''not'' bite `bar`, as `bar`'s inferred type's unification type variable is ''not'' constrained -- this is why I'm posting the example as a bug. But, I may be entirely mistaken, as the whole example makes my brain go a little fuzzy. In particular, I'm a little mystified how `MonoLocalBinds` enters the picture, but it is necessary to tickle the problem. Apologies if GHC is behaving correctly here. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9110 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler