haskell-jp / questions #50

get :: [Maybe a] -> Maybe [a]
get = undefined

some :: [Maybe Char] -> [Maybe Int] -> Maybe (Map Char Int)
some keys values = do
    k <- get keys
    v <- get values
    return Map.fromList $ zip k v

foo xs ys = sequenceA $ zipWith f xs ys
  where
    f mx my = do { x <- mx; y <- my; return (x, y) }

some :: [Maybe String] -> [Maybe Int] -> Maybe (M.Map String Int)
some keys values = do
    k <- sequence keys
    v <- sequence values
    return $ M.fromList $ zip k v

some ::　(Ord key) => [Maybe key] -> [Maybe value] -> Maybe (M.Map key value)
some keys values = do
    k <- sequence keys
    v <- sequence values
    return $ M.fromList $ zip k v

fmap M.fromList $ zipWithM (liftA2 (,)) keys values

長いリストを持っている時点でメモリ上に長いリストが展開されている

let keys = [Just (show n )| n <- [1..] ]