[GHC] #11115: Indicate missing associated type instances

[GHC]

#11115: Indicate missing associated type instances
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        Reporter:  rimmington        |                Owner:
            Type:  feature request   |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler (Type    |              Version:  7.10.2
  checker)                           |
      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 simonpj):

 That's a good idea.  But perhaps `PrefixExp Deci` isn't the villain!  It
 is part of
 {{{
   Data.Metrology.Suspicious.Poly.SplitPrefix
                                     (PrefixExp Deci) Deci Core
 }}}
 so perhaps the cause is a missing
 {{{
 type instance SplitPrefix x Deci Core = ...x....
 }}}
 I suppose that a possible heuristic would be this:
  * When reporting a "cant-match" error with `F t1 .. tn ~ ty`, where `F`
 is a type function, generate a "stuck-heuristic" message for `F t1 .. tn`.

 To generate a "stuck-heuristic" message for `F t1 .. tn`,
  * see if the call is "surely apart" from all the instances of `F`.  (See
 our closed-type family paper for "surely-apart".)
  * If so, report that there is no instance for `F t1 .. tn`.
  * If not, find a type-function application in `t1 .. tn` (say `G s1 ..
 sm`) that, if reduced, could perhaps make `F t1 .. tn` fire, if any such
 application exists.
  * And generate a stuck-heruristic message for `G s1 .. sm`

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11115#comment:1>
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