Haskell Printing Maps

store:: Map.Map String Integer
store = Map.fromList [("x", 2), ("y", 3)]

main:: IO()
main = do
    putStr "{"
    Map.foldrWithKey (\k v b -> do {putStr (k ++ " → " ++ (show v)); b}) (return()) store
    putStr "}"

-- This prints {x → 2y → 3}
-- I want {x → 2, y → 3}
-- I was trying something like this:
-- Map.foldrWithKey (\k v b -> do {putStr (k ++ " → " ++ (show v)); if b!="" then b else putStr ", "; b}) (return()) store
-- I'm not really sure how to do putStr (k ++ " → " ++ (show v)) for the first element
-- then do {putStr (", " ++ k ++ " → " ++ (show v)); b}) (return()) store for all the elements after