Re: Precedence based parsing

Jeff Kenton <Jeffrey.Kenton@comcast.net> writes:


> I have been writing parsers and compilers for over 30 years, and one
> technique I have used for parsing expressions seems to have
> disappeared from recent books. It involves comparing the precedence
> of the newest operator with that of the previous operator, in order
> to decide whether to shift or reduce. To me, it seems very
> intuitive, and easy to construct a parser by hand this way (even
> with 17 levels of operator precedence), but none of the new books
> mention it, and some very experienced colleagues have never heard of
> it. Comments? Reasons to prefer other techniques?


Some reasons operator precedence parsing has disappeared from modern
textbooks (it went away between the Aho & Ullman and the Aho, Sethi &
Ullman versions of "The Dragon Book") are:


  - It only handles very simple languages.


  - It isn't very good at discovering when a string isn't in the
      language.


  - Operator precedences have been incorporated into LR parsers, so
      writing an LR parser for expressions with many precedence levels
      isn't very hard.


  - Space isn't nearly as much an issue as it used to be.


Torben Mogensen

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