let语句中的case语句需要什么缩进？

问题描述

在haskell工作，发现了奇怪的行为，将其剥离为裸露的骨骼

此作品

  a :: Bool 
a = case 
真正 - > True 
 False  - >假
  

但是当我尝试时

  b :: IO Bool 
b = do 
 let b'=大小写真
 True  - > True 
 False  - > False 
 return b'
  

我得到

  ghci>：l test.hs 
 [1 of 1]编译Main（test.hs，解释）
 
 test.hs： 16:14：解析输入错误' - >'
失败，模块加载：无。 
  

所以我尝试

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

这是有效的。

什么？为什么？为什么在这种情况下需要额外缩进？我找不到任何东西，可能是因为这些关键字在日常语言中非常短暂和常见。是否有一些规范解释了这种行为？

解决方案

我没有准确的措辞，但是相当清楚地解释了这个问题。

  c = do 
让c'= ... 
d = ... 
e = ... 
 return c'
  

您的 True - > ... 和 False - > ... 被错误地解释为要绑定的其他变量。

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

This Works

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

But when I try

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

I get

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

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

So I try

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

And this works.

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?

解决方案

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

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'

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

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