Hi everyone,
I’m writing a racc parser, and I need to recover from parse errors.
Basically I’m writing a CSS parser, and I need to handle poorly
formatted CSS.
Unfortunately I can’t seem to find any good documentation or examples on
error recovery. I’ve read the Racc documentation about on_error and
entering “error recovering mode”, as well as calling yyerrok to leave
error recovering mode, but I don’t know what that actually means.
I’ve also discovered the “error” rule, but I don’t want to explicitly
add that rule to every rule that could possibly have an error.
Any tips would be greatly appreciated. Thanks!
I’m writing a racc parser, and I need to recover from parse errors.
…
Unfortunately I can’t seem to find any good documentation or examples on
error recovery. I’ve read the Racc documentation about on_error and
Wild stab in the dark: is there any equivalent stuff in yacc you could
use as a starting point?
I realize it probably isn’t modeled that closely, but that’s the first
thought I had.
Wild stab in the dark, part 2: tried using Sass instead? It’s a CSS
DSL. Or do you have to work with the CSS you’ve already got?
–
Giles B.
Blog: http://gilesbowkett.blogspot.com
Portfolio: http://www.gilesgoatboy.org
Tumblelog: http://giles.tumblr.com/
On Thu, Oct 25, 2007 at 02:14:20PM +0900, Giles B. wrote:
I’m writing a racc parser, and I need to recover from parse errors.
…
Unfortunately I can’t seem to find any good documentation or examples on
error recovery. I’ve read the Racc documentation about on_error and
Wild stab in the dark: is there any equivalent stuff in yacc you could
use as a starting point?
Yes there is. I was hoping I could get an answer without digging up my
lex & yacc book. 
Fortunately I found it and there is an error
recovery section that has definitely helped.
I realize it probably isn’t modeled that closely, but that’s the first
thought I had.
Wild stab in the dark, part 2: tried using Sass instead? It’s a CSS
DSL. Or do you have to work with the CSS you’ve already got?
No. I’m trying to parse CSS, not generate it.
On Oct 25, 10:29 pm, Aaron P. [email protected]
wrote:
lex & yacc book. 
Fortunately I found it and there is an error
No. I’m trying to parse CSS, not generate it.
–
Aaron P.http://tenderlovemaking.com/
Aaron,
Would you mind sharing some of what you found in the book? I’m more
interested in what you were thinking originally and then what the book
had to say to change your mind.
I’m interested in putting together some more documentation for Racc.
Mike B.
On Mon, Oct 29, 2007 at 03:40:02AM +0900, barjunk wrote:
DSL. Or do you have to work with the CSS you’ve already got?
had to say to change your mind.
What I was thinking originally was that on_error could eat up tokens
until the grammar could be recovered. Then, after reading the lex/yacc
book I found out that that is exactly what the “error” token does.
Take this grammar/script for example:
class Tester
token A LBRACE
rule
a_a_lbrace
: A A LBRACE { puts val }
| A error LBRACE { puts “got error”; puts val }
;
end
require ‘tester.tab’
class Foo < Tester
def initialize
@tokens = [
[ :A, ‘a’ ],
[ :B, ‘b’ ],
[ :LBRACE, ‘{’ ],
]
end
def parse
do_parse
end
def next_token
@tokens.shift
end
end
Foo.new.parse
The second rule gets called, and Racc uses LBRACE as a “pivot point”
(IIRC, that is what the yacc book called it). The error rule will eat
up tokens until an LBRACE is found. Then parsing can continue as usual.
Unfortunately, I’m not sure when on_error is supposed to be called. I’m
also having a hard time figuring out where to place the error token in
some of my rules. But I am going to start a new thread about that.