In Haskell, these direct equivalents aren’t bad at all: cityToState = Map.fromList [ (city, state) | (state, cities) <- Map.toList citiesByStat

In Haskell, these direct equivalents aren’t bad at all: cityToState = Map.fromList [ (city, state) | (state, cities) <- Map.toList citiesByState , city <- cities ] cityToState = Map.fromList $ do (state, cities) <- Map.toList citiesByState city <- cities pure (city, state) For multiple “nested loops”, I generally prefer do-notation over both list comprehensions and combinators such as concatMap, unless the structure is simple enough that the combinator version is much shorter.