[GHC] #13635: Incorrect result at runtime with list comprehensions in that case

[GHC]

#13635: Incorrect result at runtime with list comprehensions in that case
-------------------------------------+-------------------------------------
           Reporter:  vanto          |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.0.2
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  Incorrect result
  Unknown/Multiple                   |  at runtime
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 1. With GHC 8.0.2\\

 {{{
 Prelude> [x | x <- [_, _], False]

 <interactive>:1:12: error:
     * Found hole: _ :: t
       Where: `t' is a rigid type variable bound by
                the inferred type of it :: [t] at <interactive>:1:1
     * In the expression: _
       In the expression: [_, _]
       In a stmt of a list comprehension: x <- [_, _]
     * Relevant bindings include it :: [t] (bound at <interactive>:1:1)

 <interactive>:1:15: error:
     * Found hole: _ :: t
       Where: `t' is a rigid type variable bound by
                the inferred type of it :: [t] at <interactive>:1:1
     * In the expression: _
       In the expression: [_, _]
       In a stmt of a list comprehension: x <- [_, _]
     * Relevant bindings include it :: [t] (bound at <interactive>:1:1)
 Prelude>\\
 }}}
 This result makes no sense here.\\
 The result will always be the empty list [].\\


 2. With GHC 7.6.3\\

 {{{
 Prelude> [x | x <- [_, _], False]

 <interactive>:2:12: Pattern syntax in expression context: _

 <interactive>:2:12: Pattern syntax in expression context: _
 Prelude>
 }}}
 This result also makes no sense here.\\
 Similarly this result will always be the empty list [].

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13635>
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