Re: [GHC] #14732: -fdefer-typed-holes breaks a correct program

29 Jan 2018

      #14732: -fdefer-typed-holes breaks a correct program
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        Reporter:  MitchellSalad     |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.2.2
      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 RyanGlScott):

 You need to compile the program with `-fdefer-typed-holes` to trigger the
 error:

 {{{
 $ /opt/ghc/8.2.2/bin/ghc -fdefer-typed-holes Bug.hs
 [1 of 1] Compiling Bug              ( Bug.hs, Bug.o )

 Bug.hs:29:11: error:
     • Could not deduce (Unbox a) arising from a use of ‘zip’
       from the context: GVector Vector (a, b)
         bound by the RULE "stream/zip [Vector.Unboxed]"
         at Bug.hs:(28,11)-(30,46)
       Possible fix:
         add (Unbox a) to the context of
           the RULE "stream/zip [Vector.Unboxed]"
     • In the first argument of ‘stream’, namely ‘(zip as bs)’
       In the expression: stream (zip as bs)
       When checking the transformation rule "stream/zip [Vector.Unboxed]"
    |
 29 |   stream (zip as bs) = zipWith (,) (stream as)
    |           ^^^^^^^^^
 }}}

 This regression was introduced in 6746549772c5cc0ac66c0fce562f297f4d4b80a2
 (`Add kind equalities to GHC.`).

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14732#comment:4
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