[GHC] #10742: GHC cannot deduce (irrelevant) reflexive type equality.

Mon Aug 10 21:20:51 UTC 2015

#10742: GHC cannot deduce (irrelevant) reflexive type equality.
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        Reporter:  ntc2              |                   Owner:
            Type:  bug               |                  Status:  closed
        Priority:  normal            |               Milestone:
       Component:  Compiler (Type    |                 Version:  7.10.2
  checker)                           |                Keywords:  TypeLits
      Resolution:  fixed             |  GADTs
Operating System:  Unknown/Multiple  |            Architecture:
                                     |  Unknown/Multiple
 Type of failure:  GHC rejects       |               Test Case:
  valid program                      |  polykinds/T10742
      Blocked By:                    |                Blocking:
 Related Tickets:                    |  Differential Revisions:
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Comment (by ntc2):

 In case others run into this, here's a sketch of the workaround.

 Define a helper function to discharge the irrelevant constraint:
 {{{#!hs
 ghcBugWorkAround :: proxy m n -> ((m <=? n) ~ (m <=? n) => a) -> a
 ghcBugWorkAround _ x = x
 }}}

 Wrap the helper around the code which is causing the problem:
 {{{#!hs
 f (C ...) = <body> -- Get "can't deduce `m <= n`" error here.
 }}}
 becomes
 {{{#!hs
 f (C ...) = ghcBugWorkAround <expression fixing m and n> $ <body>
 }}}

 Note that this works even when `C ...` is a GADT pattern match which
 brings `m` or `n` into scope, in which case you can't simply change the
 type signature of `f` to include the `m <= n` constraint.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10742#comment:6>
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