[Haskell-beginners] What's wrong with my code?

Benjamin Edwards edwards.benj at gmail.com
Sun Jun 5 23:29:48 CEST 2011

On 5 June 2011 22:17, Benjamin Edwards <edwards.benj at gmail.com> wrote:

>> grahamScan points = scan (sort points)
>>             -- For each point, it is determined whether moving from the
>> two previously considered points to this point is a "left turn" or a "right
>> turn". If it is a "right turn", this means that the second-to-last point is
>> not part of the convex hull and should be removed from consideration. This
>> process is continued for as long as the set of the last three points is a
>> "right turn". As soon as a "left turn" is encountered, the algorithm moves
>> on to the next point in the sorted array. (If at any stage the three points
>> are collinear, one may opt either to discard or to report it, since in some
>> applications it is required to find all points on the boundary of the convex
>> hull.)
>>     where   scan (a : (b : (c : points)))
>>                 | (direction a b c) == TurnRight    = scan (a : (c :
>> points))
>>                 | otherwise                         = a : (scan (b : (c :
>> points)))
>>             scan (a : (b : []))                     = []
> What error message do you get? I loaded this code into ghci verbatim
> (modulo a module declaration) and it worked fine, no errors. However the
> definitions don't seem to be quite right. Consider any set of 3 non-linear
> points. Trivially the convex hull of that set is the set itself. Look at
> what your function scan does for a set of three points. It only returns 2,
> which can't be right.

Actually make that one point.
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