通过阅读Graham赫顿的《Haskell编程》（第二版），我成功地解决了练习10.5（第138页）。任务是编写一个函数adder :: IO ()，它读取n数字（交互式定义），对它们求和，并打印结果，如下所示：
> adder
How many numbers? 3
5
4
6
The total is 15函数readInt和readLine已经给出为：
readInt :: IO Int
readInt = do
line <- readLine
return (read line :: Int)
readLine :: IO String
readLine = do
c <- getChar
case c of
'\n' -> return []
_ -> do
cs <- readLine
return (c:cs)所以我只需要写adder函数：
adder :: IO ()
adder = do
putStr "How many numbers? "
n <- readInt
ns <- sequence [readInt | _ <- [1..n]]
sum <- sumUp ns
putStr $ "The total is " ++ (show sum) ++ "\n"
sumUp :: [Int] -> IO Int
sumUp xs = return $ foldl (+) 0 xs我几乎对我的解决方案很满意，但我只想内联sumUp函数。但是，我不知道该怎么做。
如何将[a] -> IO a函数内联到do块中？
1条答案
按热度按时间ugmeyewa1#
这里不需要用
return，我们可以用**sum :: (Foldable f, Num a) => f a -> a**来求和：我们也可以重复
readInt到**replicateM :: Applicative m => Int -> m a -> m [a]**：对于
readInt，这可以实现为：