Graham Scan in Haskell

gistfile1.hs

grahamScan :: [Point] -> [Point]
grahamScan pts
          | length pts < 3 = error "Not a polygon"
          | otherwise      = reverse $ scan rest (b:[a])
        where (a:b:rest)   = pts
              scan []  acc = acc
              scan pts acc = case d of
                                   TurnsRight _ _ _ -> scan ps (c:a:stack)
                                   _                -> scan ps (c:b:a:stack)
                               where (c:ps)      = sortBy sortByYThenX pts
                                     (b:a:stack) = acc
                                     d           = makeDirection a b c


-- Note that `garyScan` expects the input to be sorted via `sortByYThenX`.
garyScan :: [Point] -> [Point]
garyScan (a:b:c:rest) = case d of
                            TurnsRight _ _ _ -> garyScan (a:c:rest)
                            _                -> a : garyScan (b:c:rest)
        where d = makeDirection a b c
garyScan pts = pts