A simple text index/full text search in Haskell (I wanted a simple, but non-trivial fold example)

fulltextindex.hs

import qualified Data.Map.Strict as M
import qualified Data.Set as S

docs = ["the quick brown fox jumps over the lazy dog",
        "what does the fox say?",
        "what is the meaning of life?"]

buildIndex docs = M.unionsWith S.union [foldr (flip M.insert $ S.singleton doc) M.empty $ words doc | doc <- docs]

search _ [] = S.empty
search index query = foldr1 S.intersection $ map (flip (M.findWithDefault S.empty) index) query


main = print $ search index ["fox", "what"]
       where
         index = buildIndex docs