Expanding int maps

ExpandMap.hs

module ExpandMap
  ( expandIntMap
  ) where

import Control.Monad.State (evalState, get, put)
import Data.IntMap (IntMap)
import qualified Data.IntMap as IntMap
import Data.Traversable (for)

-- >>> expandIntMap '0' 50 (IntMap.fromList (zip [0,5..50] ['a' .. 'z']))
-- "aaaaabbbbbcccccdddddeeeeefffffggggghhhhhiiiiijjjjjk"

expandIntMap :: a -> Int -> IntMap a -> [a]
expandIntMap a n m =
  flip evalState a $
    for [0 .. n] $ \i ->
      maybe (pure ()) put (IntMap.lookup i m) *> get