extending.rst

Extending Grammars and Inheritance

warning:	Unfinished

Another feature taken from OMeta is grammar inheritance. We can write a grammar with rules that override ones in a parent. If we load the grammar from our calculator tutorial as Calc, we can extend it with some constants:

from parsley import makeGrammar
import math
import calc
calcGrammarEx = """
value = super | constant
constant = 'pi' -> math.pi
         | 'e' -> math.e
"""
CalcEx = makeGrammar(calcGrammar, {"math": math}, extends=calc.Calc)

Invoking the rule super calls the rule value in Calc. If it fails to match, our new value rule attempts to match a constant name.