如何处理多级缩进? [英] How do I deal with many levels of indentation?

问题描述

我正在编写一个具有非常复杂的逻辑循环的脚本:

I am writing a script that has a very logically complicated loop:

main = do
    inFH <- openFile "..." ReadMode
    outFH <- openFile "..." WriteMode

    forM myList $  item ->
        ...
        if ... 
            then ...
            else do
                ...
                case ... of
                    Nothing -> ...
                    Just x  -> do
                        ...
                            ...

代码很快向右飞，所以我想把它分成几部分，例如使用 where 子句.问题是，其中许多 ... 包含对 inFH 和 outFH 两个句柄的读/写语句，并使用 where 语句将使这两个名称脱离上下文.每次使用 where 语句时，我都必须发送这两个变量.

The code soon flies to the right, so I was thinking breaking it into pieces, using for example where clauses. The problem is, many of these ... contain reading/writing statements to the two handles inFH and outFH, and using a where statement will render those two names out of context. I would have to send in these two variables everytime I use a where statement.

有没有更好的方法来处理这个问题?

Is there a better way of dealing with this?

推荐答案

在很多情况下，这些深度嵌套的缩进是深度嵌套错误检查的结果.如果你是这样，你应该研究 MaybeT 和它的老大哥 ExceptT.这些提供了一种干净的方法来将出现问题时我们做什么"代码与假设一切正常我们做什么"代码分开.在你的例子中，我可能会写:

In many cases, these deeply-nested indentations are the result of deeply-nested error checking. If that's so for you, you should look into MaybeT and its big brother ExceptT. These offer a clean way to separate the "what do we do when something went wrong" code from the "what do we do assuming everything goes right" code. In your example, I might write:

data CustomError = IfCheckFailed | MaybeCheckFailed

main = handleErrors <=< runExceptT $ do
    inFH  <- liftIO $ openFile ...
    outFH <- liftIO $ openFile ...
    forM myList $ item -> do
        when (...) (throwError IfCheckFailed)
        ...
        x <- liftMaybe MaybeCheckFailed ...
        ...

liftMaybe :: MonadError e m => e -> Maybe a -> m a
liftMaybe err = maybe (throwError err) return

handleErrors :: Either CustomError a -> IO a
handleErrors (Left err) = case err of
    IfCheckFailed    -> ...
    MaybeCheckFailed -> ...
handleErrors (Right success) = return success

注意我们仍然在forM循环处增加缩进；但其他检查在 main 中内嵌"完成，并且在 handleErrors 中以相同的缩进级别处理.

Notice that we still increase indentation at the forM loop; but the other checks are done "in-line" in main, and are handled all at the same indentation level in handleErrors.

这篇关于如何处理多级缩进?的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！