Hi,

I am trying to write a partition function where we pass group sizes and the list we want to partition into groups

as arguments and get back a list of groups (or list of lists in this case). My first attempt was by using an auxiliary inner function

{-# LANGUAGE ScopedTypeVariables #-}

module Partition where

partition :: [Int] -> [a] -> [[a]]
partition ds ps = reverse $ paux ds ps []
where
  paux :: [Int] -> [a] -> [[a]] -> [[a]]
  paux [] [] ps' = ps'
  paux [] ps ps' = [ps] ++ ps’
  paux _ [] ps' = ps'
  paux (d:ds') ps ps' = paux ds' (snd (splitAt d ps)) ([fst (splitAt d ps)] ++ ps')

——————

*Partition> partition [2, 3] [1,2,3,4,5]
[[1,2],[3,4,5]]
*Partition> partition [1, 2] [1,2,3,4,5]
[[1],[2,3],[4,5]]
*Partition> partition [1, 2, 5] [1,2,3,4,5]
[[1],[2,3],[4,5]]

I was speculating if we could write the same function using foldl function but haven’t been able to figure it out.

I would really appreciate if you can give me pointers on how we can implement it.

partition' :: [Int] -> [a] -> [[a]]
partition' [] ds = [ds]
partition' ps ds = foldl ??? ???' ???''

contrary to my speculation is it even possible to write such a function using foldl if so why not?

Regards,

Apoorv Ingle
Graduate Student, Computer Science
apoorv.ingle@ku.edu