[GHC] #12393: Poor error message with equational type constraints

[GHC]

#12393: Poor error message with equational type constraints
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        Reporter:  laneb         |                Owner:
            Type:  bug           |               Status:  new
        Priority:  normal        |            Milestone:
       Component:  Compiler      |              Version:  8.0.1
      Resolution:                |             Keywords:
Operating System:  Linux         |         Architecture:  x86_64 (amd64)
 Type of failure:  None/Unknown  |            Test Case:
      Blocked By:                |             Blocking:
 Related Tickets:                |  Differential Rev(s):
       Wiki Page:                |
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Comment (by rwbarton):

 > Even if that is what's going on, the suggestion to "Use GADTs or
 TypeFamilies to permit this" is clearly wrong, as I am already using
 TypeFamilies.

 Actually, it's technically not wrong: you don't have TypeFamilies turned
 on ''in GHCi'' which is where the problematic constraint arose.

 But I certainly agree that GHC should just give the ambiguous type
 variable error in preference to the one it gave you, and probably
 shouldn't bother checking the inferred type of `it` at all.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12393#comment:1>
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