了解什么在百灵鸟语法中构成歧义

Question

我试图简化another question中提出的情况，并使用lark进行了以下解析尝试：

from lark.lark import Lark

text = """
start_thing {
  loc int {
    from 0,
    to 1093,
    strand plus,
    id gi 384632836
  }
}
"""

grammar = """\
thing: "start_thing" node
locus_info: "loc int" "{" int_info "," int_info "," STRAND_INFO "," int_info "}"
int_info: TAGS? INT
node: locus_info
    | int_info
    | TAGS? "{" nodes "}" -> subnodes
    | TAGS                -> onlytags
nodes: node?
    | node ("," node)*
STRAND_INFO: "strand" SIGN
SIGN: "plus" | "minus"
TAGS: TAGWORD (WS TAGWORD)*
TAGWORD: ("_"|LETTER)("_"|"-"|LETTER|DIGIT)*
%import common.WS
%import common.LETTER
%import common.DIGIT
%import common.INT
%ignore WS
"""

parser = Lark(grammar, start="thing", ambiguity="explicit")
parsed = parser.parse(text)
print(parsed.pretty())

输出：

thing
  subnodes
    nodes
      subnodes
        loc int
        nodes
          node
            int_info
              from
              0
          node
            int_info
              to
              1093
          onlytags  strand plus
          node
            int_info
              id gi
              384632836

如this example所示，ambiguity="explicit"选项应允许显示替代匹配的可能性，并在其前加上_ambig标签。这不会出现在上面的输出中。看来我没有什么含糊之处。

为什么“ strand plus”不被认为是模棱两可的？在我看来，它可以与STRAND_INFO或onlytags匹配。

类似地，我希望“ loc int {...}”与locus_info或subnodes匹配。