When is on_error supposed to be called? Will on_error be called when
the error rule is triggered?
After reading the lex/yacc book, it seems as though the error rule is
supposed to eat up tokens until it can find a pivot token. But is
on_error supposed to be called? I did a couple experiments and found
that on_error is not called if the error rule matches only one token,
but if it matches many tokens on_error is called.
Consider the following grammar:
class Tester
token A LBRACE
rule
a_a_lbrace
: A A LBRACE { puts val }
| A error LBRACE { puts “recovered”; puts val }
;
end
and the following script:
class Foo < Tester
def initialize
@tokens = [
[ :A, ‘a’ ],
[ :b, ‘b’ ],
[ :LBRACE, ‘{’ ],
[ false, false ],
]
end
def parse; do_parse; end
def next_token; @tokens.shift; end
def on_error(*args)
puts "on_error called"
end
end
Foo.new.parse
on_error does not get called, but the recovery rule matches. However,
change the tokens to this:
@tokens = [
[ :A, 'a' ],
[ :b, 'b' ],
[ :b, 'b' ],
[ :b, 'b' ],
[ :LBRACE, '{' ],
[ false, false ],
]
on_error gets called and the recover rule matches. It seems on_error
gets called if the error rule matches > 1 token. Is that expected
behavior? Am I using the error rule correctly?
Any help would be greatly appreciated. Thanks!