Friday, 21 May 2010

Operators, Precedence and Associativity

Expression languages often come with infix operators. For example:
x + y * z
It would be useful to be able to write a language module that abstracts the key properties of expression languages so that the module can be reused in different contexts. One of the challenges is to abstract the precedence and associativity rules in order to reflect the following two parse outcomes:
 x + (y * z) and (x + y) * z
Traditional approaches to parsing often use two approaches: (1) encode the precedence rules into the grammar rules; (2) write the parser so that it knows about certain types of operator. Both approaches have disadvantages: (1) complex grammar rules; (2) complex parsing machinery. The XPL parser, in conjunction with the language module approach, allows this to be achieved fairly painlessly without having to (1) encode the precedence in the rules; or (2) requiring the parser to know about operators.

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