haskell - 如何在 Maybe 和 IO 中使用 Do 表示法

Question

我正在努力掌握 Haskell 中的 do notation。

我可以将它与 Maybe 一起使用，然后打印结果。像这样:

maybeAdd :: Maybe Integer
maybeAdd = do one <- maybe1
              two <- maybe2
              three <- maybe3
              return (one + two + three)

main :: IO ()
main = putStr (show $ fromMaybe 0 maybeAdd)

但我没有使用单独的函数，而是尝试在 main 函数中使用带有 Maybe 的 do 表示法。但我没有任何运气。我尝试的各种尝试包括:

main :: IO ()
main = do one <- maybe1
          two <- maybe2
          three <- maybe3
          putStr (show $ fromMaybe 0 $ return (one + two + three))

main :: IO ()
main = do one <- maybe1
          two <- maybe2
          three <- maybe3
          putStr (show $ fromMaybe 0 $ Just (one + two + three))

main :: IO ()
main = do one <- maybe1
          two <- maybe2
          three <- maybe3
          putStr (show $ (one + two + three))

所有这些都会导致各种类型的编译错误，遗憾的是我未能破译以找到正确的方法。

我如何实现上述目标？也许也可以解释为什么我尝试的方法是错误的？

Answer 1

每个 do block 必须在单个 monad 中工作。如果你想使用多个 monad，你可以使用多个 do block 。尝试调整您的代码:

main :: IO ()
main = do -- IO block
   let x = do -- Maybe block
          one <- maybe1
          two <- maybe2
          three <- maybe3
          return (one + two + three)
   putStr (show $ fromMaybe 0 x)

你甚至可以使用

main = do -- IO block
   putStr $ show $ fromMaybe 0 $ do -- Maybe block
      one <- maybe1
      two <- maybe2
      three <- maybe3
      return (one + two + three)
   -- other IO actions here

但在某些情况下可能不太可读。