[GHC] #12235: Wrong compilation of bang patterns

[GHC]

#12235: Wrong compilation of bang patterns
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        Reporter:  osa1              |                Owner:
            Type:  bug               |               Status:  closed
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.1
      Resolution:  invalid           |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by nomeata):

 I agree with rwbarton. When matching `(undefined, [C])` against the
 patterns, it will match it against `(i,  [])` and fail, and then against
 `(i,  (A : ts))` and fail. Now it will be matched against `(!i, (B :
 ts))`. Comparing a tuple requires matching the individual components, from
 left to right. So it matches `undefined` against `!i`.

 According to the docs for bang patterns, as you quote them, this is done
 by first evaluating the expression to WHNF, which triggers the
 `undefined`, and this happens before `[C]` is matched against `(B:ts)`.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12235#comment:7>
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