Supermarket.hs

module Supermarket where

import Data.List
import Control.Arrow

-- Some type synonyms to clean up type signatures
type User = String
type History = [[String]]
type Map key value = [(key, value)]

-- Your dataset
users = ["user1", "user4"]
items = ["Banana", "Orange", "Kiwi", "Apple", "Pineapple", "Lemon"] 
purchasesHistory =
  [ ("user1", [ ["Banana","Orange","Kiwi"]
              , ["Banana","Orange","Apple"] ])
  , ("user4", [])]

-- Helper functions
freqList :: Ord a => [a] -> Map a Int
freqList = fmap (head &&& length) . group . sort

deleteAll :: Eq a => a -> [a] -> [a]
deleteAll x = filter (/= x)

-- Is this close to what you want to do?
getStats :: Map User History -> Map User (Map String Int)
getStats = fmap fmap fmap (fmap freqList concat)

-- This function is so you don't have to deal with `Maybe`
getCount :: Eq a => a -> Map a Int -> Int
getCount item map = case lookup item map of
  Just n  -> n
  Nothing -> 0