[GHC] #10547: feature request: expanding type synonyms in error messages

[GHC]

#10547: feature request: expanding type synonyms in error messages
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        Reporter:  osa1              |                   Owner:  osa1
            Type:  feature request   |                  Status:  new
        Priority:  low               |               Milestone:  7.12.1
       Component:  Compiler (Type    |                 Version:
  checker)                           |                Keywords:
      Resolution:                    |            Architecture:
Operating System:  Unknown/Multiple  |  Unknown/Multiple
 Type of failure:  None/Unknown      |               Test Case:
      Blocked By:                    |                Blocking:
 Related Tickets:                    |  Differential Revisions:  D1016
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Comment (by simonpj):

 Ha ha.  Yes, ideally so.  But in practice I suspect it'll be infeasible to
 find the maximally-unexpanded version; as your example shows.

 I'd settle for:
  * If the head of the application matches (i.e. `T t1` vs `T t2`) then
 walk down `t1`,`t2`.
  * Otherwise expand both sides to a non-synonym type constructor.

 There is room for cleverness here.  But a function with the above
 signature (for `expandSynonymsToMatch`) will serve the purpose, and we
 could start with a fairly simple one as above.  Or do something cleverer
 if you like.

 Simon

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10547#comment:4>
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