|parsing with user defined operators firstname.lastname@example.org (1999-08-12)|
|Re: parsing with user defined operators email@example.com (Torben Mogensen) (1999-08-13)|
|Re: parsing with user defined operators firstname.lastname@example.org (1999-08-15)|
|Re: parsing with user defined operators Andrew.Walker@nottingham.ac.uk (Dr A. N. Walker) (1999-08-27)|
|Re: parsing with user defined operators email@example.com (rspartan) (1999-08-27)|
|Re: parsing with user defined operators darcy@moa.CS.Berkeley.EDU (1999-08-28)|
|From:||Torben Mogensen <firstname.lastname@example.org>|
|Date:||13 Aug 1999 01:09:12 -0400|
Roy Germon wrote:
> I'm looking for parsing techniques to handle expressions that include
> user defined operators and overloaded operators. The precedence
> levels of operators are user definable over a range of several hundred
> values. The associativity is user definable as well. Any pointers to
> books or articles are more than welcome.
There are several ways of doing this.
1) Parse all operators with the same associativity and
precedence. The use a second pass over the syntax tree to change
the structure to reflect the precedences. This means that
parentheses must be visible in the syntax tree.
2) Define a lexical token for each possible precedence/associativity
combination. Give the lexer access to a table that contains the
precedences of all user-defined operators and use this to assign
the correct lexical token to the operator (the operator name is
now an attribute to the token).
3) Make a modified LR-parser that can resolve conflicts at
parse-time. The parser will look up the precedences in a table
when it has to resolve a conflict.
The first is the conceptually simplest method and has a small
parse-table, but requires an extra pass. The second requires a simple
fix in the lexer, something that can usually be done in the lexer
specification without having to fix the generated lexer. The
parse-table will be fairly large as it has to handle a (potentially)
large number of precedence/associativity combinations. The third
alternative requires use of a non-standard parser-generator or else a
modification of the generated parser, which may be complex. But it
uses a small parse-table and no extra pass.
Torben Mogensen (email@example.com)
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