[vector] #15: Add "generalised scan"

[vector]

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

 To give a generalised argument why this is useful: a number of dynamic
 programming algorithms work on n-dimensional data structures where all
 calculations happen on a hyperplane that moves toward a point. In 2d, we
 calculate successive diagonal entries, for example.

 Some hackary with ST and unsafeFreeze gives the same result but requires
 ST (or IO) and just doesn't look cool (and see rl's comment).

 The fact, that we calculated diagonals is unimportant, one can always
 transform the indices.

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Ticket URL: <http://trac.haskell.org/vector/ticket/15#comment:2>
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