Shape contents traversal...

Here's a bit of code using Neume's StateMap class an a Hughes list that separates the shape of data from its contents. In the code here elementary could also be coded as map (map f), but more inventive "elemenentary" functions are possible.

module ShapeContents where

import Neume.Core.Utils
import Neume.Core.Utils.HList


elementary :: (a -> b) -> [[a]] -> [[b]]
elementary f xss = reassemble (map f contents) shape
  where
    (shape, contents) = decompose xss  



decompose :: [[a]] -> ([[()]],[a])
decompose xss = fmap2b ($ []) $ stmap fn id xss 
  where
    fn stf row = (row1, stf . h1) where (row1,h1) = decompose1 row

decompose1 :: StateMap f => f a -> (f (), H a)
decompose1 = stmap fn id where
  fn stf a = ((), stf `snoc` a)
 

reassemble :: [a] -> [[()]] -> [[a]]
reassemble xs shape = fst $ stmap (stmap fn) xs shape  where
  fn (s:st) () = (s,st)
  fn _      () = error $ "reassemble - run out of data"