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Incorrect result at runtime with list comprehensions in that case

  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 [].\\

  1. 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 [].

Trac metadata
Trac field Value
Version 8.0.2
Type Bug
TypeOfFailure OtherFailure
Priority normal
Resolution Unresolved
Component Compiler
Test case
Differential revisions
BlockedBy
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Blocking
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Operating system
Architecture
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