Re: [GHC] #8450: can't match type Bool with (), but shouldn't have to

17 Oct 2013

      #8450: can't match type Bool with (), but shouldn't have to
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        Reporter:  dmwit             |            Owner:
            Type:  feature request   |           Status:  new
        Priority:  normal            |        Milestone:
       Component:  Compiler          |          Version:  7.6.3
      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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Changes (by simonpj):

 * cc: dimitris@… (added)

Comment:

 Nice example.  (The rest of this is cryptic comments mainly for
 Dimitrios.)

 The issue is this. We have a skolem 'a', and two constraints
 {{{
         [w} a ~ Bool
         [w] a ~ ()
 }}}
 But because we rewrite wanteds with wanteds, we rewrite to
 {{{
         [w] Bool ~ ()
 }}}
 which is really quite unhelpful.

 I'm beginning to wonder: what would happen if we never rewrote a wanted
 with another wanted?

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Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/8450#comment:2
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