使用PLY解析简单句子时的歧义消解问题求助

Hey there! Let's tackle this shift-reduce conflict you're hitting with your BNF parser—it's a super common headache when dealing with ambiguous grammar, especially when coordinating conjunctions like "and" are involved.

First, let's break down why your example sentence "Jared chased Tom and Jerry ate" is causing trouble. Your grammar has two overlapping rules:

1. Reducing [Noun] + "and" + [Noun] into a noun phrase
2. Reducing [Sentence] + "and" + [Sentence] into a compound sentence

The parser gets stuck at the word "and" because it has two valid options:

• Option 1: Reduce "Tom and Jerry" into a noun phrase immediately, then try to parse the rest of the sentence (which would lead to a wonky structure like "Jared chased [Tom and Jerry] ate"—semantically invalid, but grammatically allowed by your rules)
• Option 2: Shift "and" and keep parsing, waiting to form a full second sentence ("Jerry ate") to pair with the first one ("Jared chased Tom") and make a compound sentence

Here are three practical ways to fix this:

1. Rewrite Your BNF to Eliminate Ambiguity #

The cleanest fix is to tighten up your grammar rules so they don't allow conflicting interpretations. You can do this by clearly separating noun-phrase conjunctions from sentence-level conjunctions with hierarchical non-terminals:

<Sentence> ::= <SimpleSentence> | <CompoundSentence>
<CompoundSentence> ::= <SimpleSentence> "and" <SimpleSentence>
<SimpleSentence> ::= <NounPhrase> <VerbPhrase>
<VerbPhrase> ::= <TransitiveVerb> <NounPhrase> | <IntransitiveVerb>
<NounPhrase> ::= <SingleNoun> | <CoordinatedNounPhrase>
<CoordinatedNounPhrase> ::= <SingleNoun> "and" <SingleNoun>

With this setup, the parser will only reduce "X and Y" into a noun phrase if it's acting as a subject or object of a verb. When it encounters "and" after a complete simple sentence ("Jared chased Tom"), it'll know to wait for another full simple sentence instead of prematurely reducing the next pair of nouns.

2. Use Parser Priority/Associativity Rules #

If you're using a tool like Yacc/Bison, you can assign priority levels to the "and" conjunction to guide the parser's choice. Since you want sentence-level coordination to take precedence over noun-phrase coordination, you can mark the sentence-level "and" as having higher priority (or set the noun-phrase "and" to lower priority).

For example, in Bison:

%left AND  // Lower priority ensures we wait for full sentences before reducing

This tells the parser to shift "and" and continue parsing instead of immediately reducing a noun phrase when a potential sentence follow-up is available.

3. Add Semantic Checks (If Your Parser Supports It) #

If you can include semantic actions in your parser, you can add context checks during reduction. For instance, when the parser considers reducing "Tom and Jerry" to a noun phrase, it can look ahead to see if the next token is a verb that would belong to a new subject. If so, it skips the noun phrase reduction and shifts instead. This is a bit more complex, but useful if you can't rewrite your BNF for some reason.

At the end of the day, this conflict boils down to grammar ambiguity—your current rules allow two valid parse trees, and the parser doesn't know which one matches your intended semantics. By narrowing down your BNF or guiding the parser with priorities, you can eliminate the shift-reduce conflict entirely.

内容的提问来源于stack exchange，提问作者vball