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#
# left_recursion.py
#
# Example code illustrating use of left-recursion in Pyparsing.
#
import pyparsing as pp
# comment out this line to see the effects without LR parsing enabled
pp.ParserElement.enableLeftRecursion()
item_list = pp.Forward()
# a common left-recursion definition
# define a list of items as 'list + item | item'
# BNF:
# item_list := item_list item | item
# item := word of alphas
item = pp.Word(pp.alphas)
item_list <<= item_list + item | item
item_list.runTests("""\
To parse or not to parse that is the question
""")
# Define a parser for an expression that can be an identifier, a quoted string, or a
# function call that starts with an expression
# BNF:
# expr := function_call | name | string | '(' expr ')'
# function_call := expr '(' expr,... ')'
# name := Python identifier
# string := a quoted string
# from https://stackoverflow.com/questions/32809389/parse-python-code-using-pyparsing/32822575#32822575
LPAR, RPAR = map(pp.Suppress, "()")
expr = pp.Forward()
string = pp.quotedString
function_call = expr + pp.Group(LPAR + pp.Optional(pp.delimitedList(expr)) + RPAR)
name = pp.Word(pp.alphas + '_', pp.alphanums + '_')
# left recursion - call starts with an expr
expr <<= function_call | string | name | pp.Group(LPAR + expr + RPAR)
expr.runTests("""\
print("Hello, World!")
(lookup_function("fprintf"))(stderr, "Hello, World!")
""", fullDump=False)
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