[GHC] #8995: When generalising, use levels rather than global tyvars

Wed Apr 16 08:40:35 UTC 2014

#8995: When generalising, use levels rather than global tyvars
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        Reporter:  simonpj           |            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:
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Comment (by simonpj):

 Thanks Didier.

 First, two confusions:
  * I think of let-bindings of form `let x = <rhs> in <body>`.  So when I
 say "RHS" I mean the bit that you called `a2` above.  So I think we are in
 agreement here.

  * In principle GHC does not do local let-generalisation, as our papers
 advertise. But GHC must compile Haskell 98, so it must somehow support
 local let-generalisation.  So in fact:
    * Local let-gen is enabled by `-XNoMonoLocalBinds` and disabled by
 `-XMonoLocalBinds`.
    * By default, we have `-XNoMonoLocalBinds`.
    * But with `-XGADTs` or `-XTypeFamilies` we also switch on
 `-XMonoLocalBinds`.
    * But that can again be overridden, so `-XGADTs -XNoMonoLocalBings`
 would attempt to generalise local let-bindings despite the problems with
 doing so.

  The other thing is that the "monomorphism restriction" can mean that even
 a top-level  binding can have an environment with free type variables.
 `-XNoMonomorphismRestriction` switches this off.

 So this ticket is all about the "best-efforts" generalisation you get when
 you have `-XNoMonoLocalBinds`.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8995#comment:3>
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