[GHC] #13118: let binding tuple of lenses error not an expression

14 Jan 2017


      #13118: let binding tuple of lenses error not an expression
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           Reporter:  codygman       |             Owner:
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  GHCi           |           Version:  8.0.1
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 This error seems to be fixed in GHC Head +/- 10 commits, but exists in GHC
 8.0.1. I was told it might be a useful test case.


 {{{
 λ> :t userId
 userId
   :: (RElem UserId rs (RIndex UserId rs), Functor f) =>
      (Int -> f Int) -> Record rs -> f (Record rs)
 λ> let recLenses = userId &: Nil
 λ> :t recLenses
 recLenses
   :: (Functor f, RElem UserId rs (RIndex UserId rs)) =>
      Record '[s :-> ((Int -> f Int) -> Record rs -> f (Record rs))]
 λ> :t recUncons
 recUncons :: Record (s :-> a : rs) -> (a, Record rs)
 λ> :t recUncons recLenses
 recUncons recLenses
   :: (Functor f, RElem UserId rs (RIndex UserId rs)) =>
      ((Int -> f Int) -> Record rs -> f (Record rs), Record '[])
 λ> let (l, ls) = recUncons recLenses

 <no location info>: error:
     not an expression: ‘let (l, ls) = recUncons recLenses’
 }}}

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Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13118
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