Fri, 21 May 2021 03:49:43 -0700 (PDT)

Related articles |
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About finding the start symbol of a grammar ecosta.tmp@gmail.com (Eduardo Costa) (2021-05-21) |

Re: About finding the start symbol of a grammar 563-365-8930@kylheku.com (Kaz Kylheku) (2021-05-21) |

Re: About finding the start symbol of a grammar DrDiettrich1@netscape.net (Hans-Peter Diettrich) (2021-05-21) |

Re: About finding the start symbol of a grammar anton@mips.complang.tuwien.ac.at (2021-05-21) |

Re: About finding the start symbol of a grammar drikosev@gmail.com (Ev. Drikos) (2021-05-22) |

Re: About finding the start symbol of a grammar gah4@u.washington.edu (gah4) (2021-05-22) |

From: | Eduardo Costa <ecosta.tmp@gmail.com> |

Newsgroups: | comp.compilers |

Date: | Fri, 21 May 2021 03:49:43 -0700 (PDT) |

Organization: | Compilers Central |

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Keywords: | parse, question, comment |

Posted-Date: | 21 May 2021 09:46:22 EDT |

Hey guys,

I've been lately dealing with a parser generator for LL grammars, and since

it's inception I've always been blindy assuming the first element read from

within the input file is going to be the start symbol or starting rule.

So I've been wondering all this time, just out of curiosity, if there exists a

method or algorithm to find out the start symbol of a given grammar?

I guess the answer is no.

While there would exist grammars we could recursively check to find out which

it's start symbol is (i.e.: it's the only rule that used the rest of them,

where checking every other resulted in dangling rules that weren't even called

in), there might be other grammars for which more than one rule yields full

coverage (all of these obviously defining different languages) and so leading

to ambiguity.

I only contemplate a simple coverage test, even though other techniques could

exist, again, all of them leading to a point where we couldn't ascertain if

one or the other is what the user meant.

So I'm wondering if this is even an issue in production-grade

parser-generators out there?

Regards,

[yacc and its descendants have an explicit %start declaration, usually defaulting to

the first rule in the file. -John]

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