Re: [GHC] #12102: “Constraints in kin ds” illegal family application in instance ( + documentation issues?)

"GHC" <[email protected]> Wed, 06 Mar 2019 11:42:19 -0000

Newsgroups	gmane.comp.lang.haskell.glasgow.bugs
Message-ID	<[email protected]>

#12102: “Constraints in kinds” illegal family application in instance (+
documentation issues?)
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        Reporter:  Iceland_jack      |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.0.1
      Resolution:                    |             Keywords:  TypeInType
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
                                     |  typecheck/should_fail/T12102
      Blocked By:                    |             Blocking:
 Related Tickets:  #13780, #15872,   |  Differential Rev(s):  Phab:D5397
  #16263                             |
       Wiki Page:                    |
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Comment (by simonpj):

 Hang on.  The Note explains that
 * equality constraints `(~)` and `(~~)` should be fine;
 * other constraints like `Eq a` should be illegal;
 * `(~#)` isn't a constraint at all; see `Note [Types for coercions,
 predicates, and evidence]` in `TyCoRep`

 We should both make the user manual reflect this, and fix any bugs (which
 surely exist).

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12102#comment:21>