在where子句下执行语句 [英] Do statement under a where clause

问题描述

我正在尝试使用 <- 绑定将 IO [String] 转换为[String]；但是，我需要使用do块在where语句下执行此操作，但是Haskell始终抱怨缩进.这是代码:

I'm trying to convert IO [String] to [String] with <- binding; however, I need to use a do block to do that under a where statement, but Haskell complains about the indentation all the time. Here is the code:

decompEventBlocks :: IO [String] -> IO [[String]]
decompEventBlocks words
 | words' /= [] = block : (decompEventBlocks . drop $ (length block) words')
 | otherwise = []
  where 
   do
    words' <- words
    let block = (takeWhile (/="END") words')

这是什么原因?以及如何在 where 语句中使用 do 块?而且，我们是否有可能在警卫人员面前发表一些声明?

What is the reason for that ? And how can we use do block in a where statement ? Moreover, is there any chance that we can have some statements before the guards ?

推荐答案

记住:do-块是语法糖表示单子符号.这意味着以下情况适用:

Remember: do-blocks are syntactic sugar for monadic notation. This means the following applies:

do {a; b} = a >> b
dp {a <- b; c} = b >>= \a -> c

换句话说，当使用do表示法时，实际上是在生成值.这就是为什么您不能在where语句的顶层仅包含do -block的原因.

In other words, when using do-notation, you are actually producing values. This is why you can't just have a do-block in the top level of your where statement.

解决此问题的方法是将函数放入do -block中:

The way to solve this is to put the function into a do-block:

decompEventBlocks :: IO [String] -> IO [[String]]
decompEventBlocks words = do
    -- We unwrap the IO [String], but we keep it in the do-block,
    -- because it must be kept in a monadic context!
    words' <- words 
    let block = (takeWhile (/="END") words')
    -- This is equivalent to the guards you had in your function.
    -- NB return :: Monad m => a -> m a, to keep it in a monadic context!
    if not $ null words'
        then do 
          -- Since the recursion is monadic, we must bind it too:
          rest <- decompEventBlocks $ return $ drop (length block) words'
          return $ block : rest
        else return []

要了解有关monad，do-符号，>>=和>>的信息，我强烈建议阅读 LYAH章节，以便在尝试更多单子代码之前获得良好的理解.

To learn about monads, do-notation, >>=, and >>, I highly reccommend reading the LYAH chapters to gain a good understanding before attempting more monadic code.

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