GeneralFoldDavidYoung.hs

-- GeneralFoldDavidYoung.hs

generalFold :: ((i,o) -> Either o (i,o)) -> (i,o) -> o
generalFold fn initial = let Left result = go initial in result
 where go x = fn x >>= go

sum5 = generalFold transition ([1..5],0)
 where
  transition (list,acc) =
   if list == [] then Left acc
   else Right (tail list,(head list)+acc)

product5 = generalFold transition ([1..5],1)
 where
  transition (list,acc) =
   if list == [] then Left acc
   else Right (tail list,(head list)*acc)

vectorToMatrix :: [Double] -> Int -> [[Double]]
vectorToMatrix vector nColumns = generalFold transition (vector, [])
 where
  transition (info, acc)
   | n < 0 = Left acc
   | otherwise = Right (take n info, drop n info : acc)
   where n = length info - nColumns

matrix3x3 = vectorToMatrix [1..9] 3

-- Now, if you want to keep the last state of the input, use this:

generalFold2 :: ((i,o) -> Either (i,o) (i,o)) -> (i,o) -> (i,o)
generalFold2 fn initial = let Left result = go initial in result
 where go x = fn x >>= go

vectorToMatrix2 :: [Double] -> Int -> ([Double],[[Double]])
vectorToMatrix2 vector nColumns = generalFold2 transition (vector, [])
 where
  transition (info, acc)
   | n < 0 = Left (info,acc)
   | otherwise = Right (take n info, drop n info : acc)
   where n = length info - nColumns

matrix2 = vectorToMatrix2 [1..10] 3
-- matrix2 == ([1.0],[[2.0,3.0,4.0],[5.0,6.0,7.0],[8.0,9.0,10.0]])