foo.hs

module Foo where

import Control.Applicative
import Data.Set (Set)
import Data.Map (Map)
import qualified Data.Map as M
import qualified Data.Set as S

get :: String -> IO (Maybe String)
get = undefined

multiGet :: Set String -> IO (Map String String)
multiGet = undefined

test :: IO (Maybe String, Maybe String, Maybe String)
test = (,,) <$> get "foo" <*> get "bar" <*> get "baz"

unbatch :: Map String a -> (Maybe a, Maybe a, Maybe a)
unbatch = (,,) <$> M.lookup "foo" <*> M.lookup "bar" <*> M.lookup "baz"

fetch :: IO (Map String String)
fetch = multiGet $ S.fromList ["foo", "bar", "baz"]

testBatch :: IO (Maybe String, Maybe String, Maybe String)
testBatch = unbatch <$> fetch