[GHC] #13547: Lint error in arrows program

[GHC]

#13547: Lint error in arrows program
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        Reporter:  cipher1024        |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.0.1
  (CodeGen)                          |
      Resolution:                    |             Keywords:  Arrows
Operating System:  Unknown/Multiple  |         Architecture:  x86_64
 Type of failure:  Compile-time      |  (amd64)
  crash or panic                     |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  10158             |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by RyanGlScott):

 cipher1024, is my understanding of arrows correct in that the arrow
 notation you use in `step` would desugar down to something like this?

 {{{#!hs
 step :: LatexParserA Cell1 r
 step = arr (\prxy -> prxy) >>> id >>> arr (\(Cell prxy') -> prxy') >>>
 stepList
 }}}

 If so, the Core Lint error is somewhat understandable, as trying to
 typecheck that code simply won't work:

 {{{
 Bug.hs:25:61: error:
     • Couldn't match type ‘rule0’ with ‘a’
         because type variable ‘a’ would escape its scope
       This (rigid, skolem) type variable is bound by
         a pattern with constructor: Cell :: forall a. Proxy a -> Cell1,
         in a lambda abstraction
         at Bug.hs:25:46-55
       Expected type: Proxy rule0
         Actual type: Proxy a
     • In the expression: prxy'
       In the first argument of ‘arr’, namely ‘(\ (Cell prxy') -> prxy')’
       In the first argument of ‘(>>>)’, namely
         ‘arr (\ (Cell prxy') -> prxy')’
     • Relevant bindings include
         prxy' :: Proxy a (bound at Bug.hs:25:51)
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13547#comment:31>
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