在Ruby中使用Parslet进行缩进敏感的解析器? [英] Indentation sensitive parser using Parslet in Ruby?

问题描述

我正在尝试使用Ruby中的 Parslet 库解析一种简单的对缩进敏感的语法.

I am attempting to parse a simple indentation sensitive syntax using the Parslet library within Ruby.

以下是我尝试解析的语法示例:

The following is an example of the syntax I am attempting to parse:

level0child0
level0child1
  level1child0
  level1child1
    level2child0
  level1child2

生成的树看起来像这样:

The resulting tree would look like so:

[
  {
    :identifier => "level0child0",
    :children => []
  },
  {
    :identifier => "level0child1",
    :children => [
      {
        :identifier => "level1child0",
        :children => []
      },
      {
        :identifier => "level1child1",
        :children => [
          {
            :identifier => "level2child0",
            :children => []
          }
        ]
      },
      {
        :identifier => "level1child2",
        :children => []
      },
    ]
  }
]

我现在拥有的解析器可以解析嵌套级别0和1的节点，但是不能解析过去的嵌套:

The parser that I have now can parse nesting level 0 and 1 nodes, but cannot parse past that:

require 'parslet'

class IndentationSensitiveParser < Parslet::Parser

  rule(:indent) { str('  ') }
  rule(:newline) { str("\n") }
  rule(:identifier) { match['A-Za-z0-9'].repeat.as(:identifier) }

  rule(:node) { identifier >> newline >> (indent >> identifier >> newline.maybe).repeat.as(:children) }

  rule(:document) { node.repeat }

  root :document

end

require 'ap'
require 'pp'

begin
  input = DATA.read

  puts '', '----- input ----------------------------------------------------------------------', ''
  ap input

  tree = IndentationSensitiveParser.new.parse(input)

  puts '', '----- tree -----------------------------------------------------------------------', ''
  ap tree

rescue IndentationSensitiveParser::ParseFailed => failure
  puts '', '----- error ----------------------------------------------------------------------', ''
  puts failure.cause.ascii_tree
end

__END__
user
  name
  age
recipe
  name
foo
bar

很明显，我需要一个动态计数器，该计数器希望3个缩进节点与嵌套级别3上的标识符匹配.

It's clear that I need a dynamic counter that expects 3 indentation nodes to match a identifier on the nesting level 3.

如何以这种方式使用Parslet实现缩进敏感的语法解析器?有可能吗?

How can I implement an indentation sensitive syntax parser using Parslet in this way? Is it possible?

推荐答案

有几种方法.

1. 通过将每一行识别为缩进和标识符的集合来解析文档，然后再进行转换以根据缩进的数量重建层次结构.

1. Parse the document by recognising each line as a collection of indents and an identifier, then apply a transformation afterwards to reconstruct the hierarchy based on the number of indents.

使用捕获来存储当前的缩进，并期望下一个节点包含该缩进以及其他要与之匹配的缩进(我不太喜欢这种方法，因为下一个发生在我身上)

Use captures to store the current indent and expect the next node to include that indent plus more to match as a child (I didn't dig into this approach much as the next one occurred to me)

规则只是方法.因此，您可以将"node"定义为方法，这意味着您可以传递参数！ (如下)

Rules are just methods. So you can define 'node' as a method, which means you can pass parameters! (as follows)

这使您可以根据node(depth+1)定义node(depth).但是，这种方法的问题在于node方法与字符串不匹配，它生成了一个解析器.因此，递归调用将永远不会结束.

This lets you define node(depth) in terms of node(depth+1). The problem with this approach, however, is that the node method doesn't match a string, it generates a parser. So a recursive call will never finish.

这就是为什么dynamic存在的原因.它会返回一个解析器，直到解析器尝试匹配它为止，该解析器无法解析，从而使您现在可以毫无问题地进行递归操作.

This is why dynamic exists. It returns a parser that isn't resolved until the point it tries to match it, allowing you to now recurse without problems.

请参见以下代码:

require 'parslet'

class IndentationSensitiveParser < Parslet::Parser

  def indent(depth)
    str('  '*depth)
  end

  rule(:newline) { str("\n") }

  rule(:identifier) { match['A-Za-z0-9'].repeat(1).as(:identifier) }

  def node(depth) 
    indent(depth) >> 
    identifier >> 
    newline.maybe >> 
    (dynamic{|s,c| node(depth+1).repeat(0)}).as(:children)
  end 

  rule(:document) { node(0).repeat }

  root :document
end

这是我最喜欢的解决方案.

This is my favoured solution.

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