[GHC] #12593: ghci spins forever

[GHC]

#12593: ghci spins forever
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        Reporter:  Iceland_jack      |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.0.1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by Simon Peyton Jones <simonpj@…>):

 In [changeset:"66a8c194520aadcaa0482736f3fdd4d2f31a5586/ghc" 66a8c19/ghc]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="66a8c194520aadcaa0482736f3fdd4d2f31a5586"
 Fix a bug in occurs checking

 1. Trac #12593 exposed a long-standing bug in the occurs
    checking machinery.  When unifying two type variables
           a ~ b
    where a /= b, we were assuming that there could be
    no occurs-check error.  But there can: 'a' can occur
    in b's kind!  When the RHS was a non-tyvar we used
    occurCheckExpand, which /did/ look in kinds, but not
    when the RHS was a tyvar.

    This bug has been lurking ever since TypeInType, maybe
    longer.  And it was present both in TcUnify (the on-the-fly
    unifier), and in TcInteract.

    I ended up refactoring both so that the tyvar/tyvar
    path naturally goes through the same occurs-check as
    non-tyvar rhss.  It's simpler and more robust now.

    One good thing is that both unifiers now share
        TcType.swapOverVars
        TcType.canSolveByUnification
    previously they had different logic for the same goals

 2. Fixing this bug exposed another!  In T11635 we end
    up unifying
    (alpha :: forall k. k->*) ~ (beta :: forall k. k->*)
    Now that the occurs check is done for tyvars too, we
    look inside beta's kind.  And then reject the program
    becuase of the forall inside there.  But in fact that
    forall is fine -- it does not count as impredicative
    polymoprhism.   See Note [Checking for foralls]
    in TcType.

 3. All this fuss around occurrence checking forced me
    to look at TcUnify.checkTauTvUpdate
           and TcType.occurCheckExpand
    There's a lot of duplication there, and I managed
    to elminate quite a bit of it. For example,
    checkTauTvUpdate called a local 'defer_me'; and then
    called occurCheckExpand, which then used a very
    similar 'fast_check'.

    Things are better, but there is more to do.
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12593#comment:4>
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