Haskell“哪里"缩进:为什么它必须缩进过去的标识符? [英] Haskell "where" indentation: why must it be indented past identifier?

问题描述

此代码:

import Data.Char (digitToInt)

myInt :: String -> Int
myInt [] = error "bad input: empty string"
myInt (x:xs)
  | x == '-'  = -1 * myInt xs
  | otherwise = foldl convert 0 (x:xs)
  where convert acc x
        | x `elem` ['0'..'9'] = 10 * acc + digitToInt x
        | otherwise           = error ("bad input: not an int - " ++ [x])

失败:

前奏> :l safeListFs.hs
[1 of 1] 编译 Main(safeListFs.hs，已解释)

Prelude> :l safeListFs.hs
[1 of 1] Compiling Main ( safeListFs.hs, interpreted )

safeListFs.hs:9:8:解析错误(可能缩进不正确)
失败，加载的模块:无.

safeListFs.hs:9:8: parse error (possibly incorrect indentation)
Failed, modules loaded: none.

但是这个版本:

import Data.Char (digitToInt)

myInt :: String -> Int
myInt [] = error "bad input: empty string"
myInt (x:xs)
  | x == '-'  = -1 * myInt xs
  | otherwise = foldl convert 0 (x:xs)
  where convert acc x
          | x `elem` ['0'..'9'] = 10 * acc + digitToInt x
          | otherwise           = error ("bad input: not an int - " ++ [x])

没问题:

前奏> :l safeListFs.hs
[1 of 1] 编译 Main(safeListFs.hs，已解释)
好的，加载的模块:Main.

Prelude> :l safeListFs.hs
[1 of 1] Compiling Main ( safeListFs.hs, interpreted )
Ok, modules loaded: Main.

我不明白为什么最后两个缩进很重要.

I can't figure out why those two last indents matter.

推荐答案

基本上，Haskell 会注意到 where 之后第一个非空格字符出现的列(在这种情况下，c of convert) 并将从该列开始的以下几行视为 where 中的新定义.

Basically, Haskell notes the column where the first non-space character after where appears (in this case, the c of convert) and treats following lines beginning in that column as new definitions inside the where.

继续前一行定义的行(例如您的 | 守卫)必须缩进第一个非空格字符(|>c 在您的代码中).

A line that continues the definition of the previous line (such as your | guards) must be indented to the right of the first non-space character (c in your code).

在 c 左侧缩进的行将在 where 之外(例如，下一个顶级函数的开始).

A line indented to the left of c would be outside the where (for example, the start of your next top-level function).

where 之后的第一个字符的列是至关重要的，即使它在一个新行上:

It's the column of the first character following where that is crucial, even if it's on a new line:

  where
    convert acc x
      | ...
    anotherFunction x y

    ^ 

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