我如何处理多级缩进？

Question

我正在编写一个具有非常复杂的循环的脚本:

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

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

代码很快就会飞到右边,所以我想把它分成几块,使用例如where子句.问题是,许多这些...包含读/写语句到两个把手inFH和outFH,使用where的语句将呈现这两个名字断章取义.我每次使用where语句时都必须发送这两个变量.

有没有更好的方法来解决这个问题？

Answer 1

在许多情况下,这些深度嵌套的缩进是深度嵌套错误检查的结果.如果那样对你来说,你应该调查MaybeT它的大哥ExceptT.这些代码提供了一种干净的方法来将"我们做什么时出错"代码与"假设一切正常我们做什么"代码分开.在您的示例中,我可能会写:

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

请注意,我们仍然在forM循环中增加缩进; 但其他检查是"在线"完成的main,并且在所有相同的缩进级别处理handleErrors.

Answer 2

虽然可能有更好的方法来解决您的具体问题(参见Daniel Wagner的答案),但您始终可以使用let在任意范围内引入新名称.这是一个公认的荒谬的演示:

main = do
    inFH <- return "inf"
    outFH <- return "ouf"

    let subAction = do
            if length inFH > 2
                then print "foo"
                else subSubAction

        subSubAction = case outFH of
            [] -> print "bar"
            _ -> print "baz"

    forM [1..10] $ \ item -> do
        print item
        subAction