let 语句中的 case 语句需要什么缩进? [英] What indentation is required for a case statement within a let statement?

问题描述

在 Haskell 中工作，发现奇怪的行为，将其剥离为裸露的骨头

Working in haskell, found odd behavior, stripped it down to bare bones

这个作品

a :: Bool
a = case True of
    True -> True
    False -> False

但是当我尝试时

b :: IO Bool
b = do
    let b' = case True of
        True -> True
        False -> False
    return b'

我明白

ghci>:l test.hs
[1 of 1] Compiling Main             ( test.hs, interpreted )

test.hs:16:14: parse error on input ‘->’
Failed, modules loaded: none.

所以我试试

c :: IO Bool
c = do
    let c' = case True of
            True -> True
            False -> False
    return c'

这行得通.

什么?为什么?为什么在这种情况下我需要额外的缩进?我找不到任何关于此的信息，可能是因为这些关键字在日常语言中是如此简短和常见.是否有一些规范可以解释这种行为?

What? Why? Why do I need an extra indent in this case? I can't find anything on this, probably because these keyword are so short and common in everyday language. Is there some spec that explains this behavior?

推荐答案

我没有规范中的确切措辞，但是这个 Wikibook page 非常清楚地解释了这个问题.

I don't have the exact wording from the spec, but this Wikibook page explains the issue quite clearly.

这样做的原因很简单:支持通过单个let-group绑定多个变量，例如:

The reason why it works like this is simple: to support binding multiple variables via a single let-group, such as:

c = do
    let c' = …
        d  = …
        e  = …
    return c'

你的 True ->... 和 False ->... 被错误地解释为要绑定的附加变量.

Your True -> … and False -> … are mistakenly interpreted as additional variables to be bound.

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