[GHC] #9110: MonomorphismRestriction too eager

[GHC]

#9110: MonomorphismRestriction too eager
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       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:                    |
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 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>
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