Very nice. Care for some unsolicited hlint-like suggestions? Why not simply this: img <- readImage "file.png" -- instead of this? t <- readImage

Very nice. Care for some unsolicited hlint-like suggestions? Why not simply this: img <- readImage "file.png" -- instead of this? t <- readImage "file.png" let img = t Why not this: map (y!!) x foldr ((:).(y!!)) []　x　-- instead of this? Note that a way of reading fold g [] is that replaces the list constructor (:) (aka cons) with g. So (:) as argument of fold is probably not idiomatic, since foldr (:) [] is the same as id.