Split list into chunks of 2 or 3 elements

split.hs

split :: [a] -> [[a]]
split [a,b,c,d]  = [a,b] : [c,d] : []
split (a:b:c:xs) = [a,b,c] : split xs
split []         = []
split xs         = xs : []

split.sml

fun loop (xs, a :: b :: c :: d :: nil) = (c :: d :: nil) :: (a :: b :: nil) :: xs
  | loop (xs, a :: b :: c :: x)        = loop ((a :: b :: c :: nil) :: xs, x)
  | loop (xs, nil)                     = xs
  | loop (xs, x)                       = x :: xs

fun split xs = rev (loop (nil, xs))