[vector] #15: Add "generalised scan"

[vector]

#15: Add "generalised scan"
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Reporter:  rl           |        Owner:     
    Type:  enhancement  |       Status:  new
Priority:  critical     |    Milestone:  0.7
 Version:  0.6          |   Resolution:     
Keywords:               |  
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Comment (by rl):

 Indeed, querying the length is just as fast in this case so there is no
 reason to pass it separately. I'm still not entirely sure how to best
 implement this for boxed vectors which is the only reason it isn't in the
 repository yet. The next vector release will include this, though.

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Ticket URL: <http://trac.haskell.org/vector/ticket/15#comment:5>
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