haskell - 了解 `sequence_`

Question

All About Monads解释sequence_:

The sequence_ function (notice the underscore) has the same behavior as sequence but does not return a list of results. It is useful when only the side-effects of the monadic computations are important.

然后，查看TestSequence.hs:

import Control.Monad

f :: String -> IO ()
f x = print x

run :: [String] -> IO ()
run xs = sequence_ . map f $ xs

我可以运行它:

λ: run ["foo", "bar"]
"foo"
"bar"

sequence_ 是否在每个 IO () 上调用 unsafePerformIO 来获取结果，即 ()？

而且，sequence_ 是否不受欢迎？或者对于 IO Monad 来说，只是简单地使用“在世界末日”来运行一系列 IO 操作？

Answer 1

不，它不会在每个 IO () 操作上调用 unsafePerformIO。事实上，它的类型甚至不特定于 IO:

sequence_ :: (Monad m, Foldable t) => t (m a) -> m ()

在旧的库中，当它特定于列表(而不是对所有 Foldable 通用)时，它是通过以下完全可读的方式实现的:

sequence_ [] = return ()
sequence_ (x:xs) = x >> sequence_ xs

绝对不气馁； sequence_(及其老大哥 mapM_)非常有用，以至于它甚至是我解释为什么 Monad 的激励示例之一作为一个抽象是有用的。