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Contents
# A demonstration of the new precedence climbing infix expression parser.
$:.unshift File.dirname(__FILE__) + "/../lib"
require 'pp'
require 'rspec'
require 'parslet'
require 'parslet/rig/rspec'
require 'parslet/convenience'
class InfixExpressionParser < Parslet::Parser
root :variable_assignment_list
rule(:space) { match[' '] }
def cts atom
atom >> space.repeat
end
def infix *args
Infix.new(*args)
end
# This is the heart of the infix expression parser: real simple definitions
# for all the pieces we need.
rule(:mul_op) { cts match['*/'] }
rule(:add_op) { cts match['+-'] }
rule(:digit) { match['0-9'] }
rule(:integer) { cts digit.repeat(1).as(:int) }
rule(:expression) { infix_expression(integer,
[mul_op, 2, :left],
[add_op, 1, :right]) }
# And now adding variable assignments to that, just to a) demonstrate this
# embedded in a bigger parser, and b) make the example interesting.
rule(:variable_assignment_list) {
variable_assignment.repeat(1) }
rule(:variable_assignment) {
identifier.as(:ident) >> equal_sign >> expression.as(:exp) >> eol }
rule(:identifier) {
cts (match['a-z'] >> match['a-zA-Z0-9'].repeat) }
rule(:equal_sign) {
cts str('=') }
rule(:eol) {
cts(str("\n")) | any.absent? }
end
class InfixInterpreter < Parslet::Transform
rule(int: simple(:int)) { Integer(int) }
rule(ident: simple(:ident), exp: simple(:result)) { |d|
d[:doc][d[:ident].to_s.strip.to_sym] = d[:result] }
rule(l: simple(:l), o: /^\*/, r: simple(:r)) { l * r }
rule(l: simple(:l), o: /^\+/, r: simple(:r)) { l + r }
end
input = <<ASSIGNMENTS
a = 1
b = 2
c = 3 * 25
d = 100 + 3*4
ASSIGNMENTS
puts input
int_tree = InfixExpressionParser.new.parse_with_debug(input)
bindings = {}
result = InfixInterpreter.new.apply(int_tree, doc: bindings)
pp bindings
Version data entries
11 entries across 11 versions & 2 rubygems