12 Nov 2005 16:40:12 -0500

Related articles |
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What is correct way to describe this in BNF for an LL(1) parser donmackay@optushome.com.au (don) (2005-11-02) |

Re: What is correct way to describe this in BNF for an LL(1) parser mailbox@dmitry-kazakov.de (Dmitry A. Kazakov) (2005-11-04) |

What is correct way to describe this in BNF for an LL(1) parser rici@ricilake.net (Rici Lake) (2005-11-04) |

Re: What is correct way to describe this in BNF for an LL(1) parser donmackay@optushome.com.au (don) (2005-11-08) |

Re: What is correct way to describe this in BNF for an LL(1) parser henry@spsystems.net (2005-11-12) |

From: | henry@spsystems.net (Henry Spencer) |

Newsgroups: | comp.compilers |

Date: | 12 Nov 2005 16:40:12 -0500 |

Organization: | SP Systems, Toronto, Canada |

References: | 05-11-032 05-11-038 |

Keywords: | parse, LL(1) |

Posted-Date: | 12 Nov 2005 16:40:12 EST |

Rici Lake <rici@ricilake.net> wrote:

*>mult-expr ::= pow-expr '/' mult-expr*

*> | pow-expr '*' mult-expr*

*> | pow-expr mult-expr ;; implicit multiplication*

*>*

*>term ::= '|' expr '|'*

*> | '(' expr ')'*

*> | VAR*

*> | NUMBER*

*>*

*>(I don't know what tool you are using or any details of your grammar;*

*>this is just a guess. Make appropriate substitutions, or ignore me if*

*>I'm guessing wrong.)*

*>*

*>If it were not for implicit multiplication, the use of |x| for abs(x)*

*>is unambiguous (providing you're not using | for other things as well...*

It can be unambiguous even with implicit multiplication, at the cost of

a bit more grammar complexity. Consider (using RFC 2234 ABNF):

expr = term

/ expr term ; implicit multiplication

term = "|" aexpr "|"

/ aterm

aexpr = aterm

/ aexpr aterm

aterm = "(" expr ")"

/ VAR

/ NUMBER

This excludes nested absolute-value operators unless the inner ones are

enclosed in parentheses. With a full set of operators it would get

tedious, since it essentially duplicates the expression syntax with one

form excluded, but it *does* work.

--

spsystems.net is temporarily off the air; | Henry Spencer

mail to henry at zoo.utoronto.ca instead. | henry@spsystems.net

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