使用 foldLine 解析多个 block

Question

对于这个简化的问题，我试图解析一个看起来像这样的输入

foo bar
 baz quux 
 woo
hoo xyzzy 
  glulx

进入

[["foo", "bar", "baz", "quux", "woo"], ["hoo", "xyzzy", "glulx"]]

我试过的代码如下:

import qualified Text.Megaparsec.Lexer as L
import Text.Megaparsec hiding (space)
import Text.Megaparsec.Char hiding (space)
import Text.Megaparsec.String
import Control.Monad (void)
import Control.Applicative

space :: Parser ()
space = L.space (void spaceChar) empty empty

item :: Parser () -> Parser String
item sp = L.lexeme sp $ some letterChar

items :: Parser () -> Parser [String]
items sp = L.lineFold sp $ \sp' -> some (item sp')

items_ :: Parser [String]
items_ = items space

这适用于一 block items:

λ» parseTest items_ "foo bar\n baz quux\n woo"
["foo","bar","baz","quux","woo"]

但是当我尝试解析许多项目时，它在第一个未缩进的行上失败了:

λ» parseTest (many items_) "foo bar\n baz quux\n woo\nhoo xyzzy\n  glulx"
4:1:
incorrect indentation (got 1, should be greater than 1)

或者，使用更简单的输入:

λ» parseTest (many items_) "a\nb"
2:1:
incorrect indentation (got 1, should be greater than 1)

Answer 1

Megaparsec 的作者在这里 :-) 使用时要记住一件事Megaparsec 是它的词法分析器模块是故意“低级”的。它不做任何你不能建立自己的事情，它不会把你锁在任何特定的“框架”。所以基本上在你的情况下你有空间消费者sp' 为您提供，但您应该小心使用它，因为它肯定当您的缩进级别小于或等于缩进级别时失败整个折叠的开始，顺便说一句，这就是你的折叠结束的方式。

引用the docs :

Create a parser that supports line-folding. The first argument is used to consume white space between components of line fold, thus it must consume newlines in order to work properly. The second argument is a callback that receives custom space-consuming parser as argument. This parser should be used after separate components of line fold that can be put on different lines.

sc = L.space (void spaceChar) empty empty

myFold = L.lineFold sc $ \sc' -> do
  L.symbol sc' "foo"
  L.symbol sc' "bar"
  L.symbol sc  "baz" -- for the last symbol we use normal space consumer

Line fold 不能无限期地运行，所以你应该预料到它会因错误而失败类似于您现在拥有的消息。要想成功，你应该思考关于它完成的方法。这通常是通过使用“正常”来完成的行尾的空间消费者折叠:

space :: Parser ()
space = L.space (void spaceChar) empty empty

item :: Parser String
item = some letterChar

items :: Parser () -> Parser [String]
items sp = L.lineFold sp $ \sp' ->
  item `sepBy1` try sp' <* sp

items_ :: Parser [String]
items_ = items space

item `sepBy1` try sp' 运行直到失败，然后 sp 获取其余部分，所以可以解析下一个折叠。

λ> parseTest items_ "foo bar\n baz quux\n woo"
["foo","bar","baz","quux","woo"]
λ> parseTest (many items_) "foo bar\n baz quux\n woo\nhoo xyzzy\n  glulx"
[["foo","bar","baz","quux","woo"],["hoo","xyzzy","glulx"]]
λ> parseTest (many items_) "foo bar\n baz quux\n woo\nhoo\nxyzzy\n  glulx"
[["foo","bar","baz","quux","woo"],["hoo"],["xyzzy","glulx"]]