menu-items-gist.hs

import qualified Data.Map as M (Map, empty, insertWith, toList)
import Data.Foldable
import Data.List

example_csv = "rest1,item1\nrest1,item2\nrest3,item1\nrest3,item2\nrest2,item3\nrest1,item5\nrest2,item2"

csvToDict :: String -> M.Map String [String]
csvToDict = foldToDict . map splitOnComma . lines
  where
    splitOnComma = words . map (\c -> if c == ',' then ' ' else c)
    foldToDict = foldr (\[rest, item] -> M.insertWith (++) rest [item]) M.empty

main = do
  let mapping = sort . M.toList . csvToDict $ example_csv
  forM_ mapping $ \(rest, items) -> do
    putStrLn rest
    forM_ (sort items) $ \i -> putStrLn $ '\t' : i