在where子句下做声明

Question

我想转换IO [String]到[String]具有<-约束力.但是,我需要do在一个where声明中使用一个块来做到这一点,但是Haskell一直抱怨缩进.这是代码:

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？我们如何do在where声明中使用块？而且,我们是否有机会在警卫面前发表一些声明？

Answer 1

您无法将 IO String 转换为String.

但是,您可以做的是将IO String的内容绑定到'变量',但这仍然会导致整个计算嵌入到IO中.

foo = do
   x <- baz -- here baz is the IO String
   let x' = doStuff x
   return x' -- embeds the String inside IO, as otherwise the computation would result in IO ()

回答你的问题

foo x = baz x -- x here is your 'IO String'
  where
    baz x = do
      x' <- x
      return $ doStuff x'