[GHC] #14367: Lazy evaluation can be invalidated

[GHC]

#14367: Lazy evaluation can be invalidated
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        Reporter:  vanto             |                Owner:  (none)
            Type:  bug               |               Status:  closed
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.2.1
      Resolution:  invalid           |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  Other             |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by int-e):

 There is a curiosity here, IMHO:
 {{{
 GHCi, version 8.2.1: http://www.haskell.org/ghc/  :? for help
 Prelude> :set -fdefer-typed-holes -Wno-typed-holes
 Prelude> :set -fdefer-out-of-scope-variables -Wno-deferred-out-of-scope-
 variables
 Prelude> let x = [_]; y = [r]
 Prelude> (length x, length y)
 (1,1)
 Prelude> length [_]

 <interactive>:4:9: error:
     • Found hole: _ :: a0
       Where: ‘a0’ is an ambiguous type variable
     • In the expression: _
       In the first argument of ‘length’, namely ‘[_]’
       In the expression: length [_]
     • Relevant bindings include it :: Int (bound at <interactive>:4:1)
 Prelude> length [r]
 1
 }}}
 Why does the `length [_]` expression produce a compile time error?

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14367#comment:3>
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