When Rageling, Bring a Bird Beak #
While bunking inside Ragel, I’m starting to see the handiness of its lookahead operator. The bird beak :>, which gives a high priority to the next token in the machine.
In SuperRedCloth, this rule here was a flimsy way of matching a bit of text in parens:
title = ( '(' [^)]+ ')' );
It turns out I was able to cut down the generated grammer by something like fifty K by using this instead:
title = ( '(' chars+ :> ')' ) ;
The beak is great for any rule that’s getting too greedy and concludes with an exact character. Be warned, it may increase the complexity of the machine depending on how ambiguous your other rules are.
If I could offer one other bit of advice: stay away from the question mark, if you can. It seems like anything which uses the empty token can be expensive.
MenTaLguY
Let’s also not forget the evil twin brother of our be-beaked friend, that sinister beaky bird, greedy as can be:
<:MenTaLguY
Adrian set me straight on one thing with optionality and priorities, by the way. For most things,
expr1 :> expr2 expr3works fine, but not ifexpr2can be empty somehow.Thing is,
>:and friends can peel off an optional machine like teflon. You’ve really got to doexpr1 :> (expr2? expr3)instead ofexpr1 :> expr2? expr3.Once you add that question mark, you’ve got some transitions jumping straight from
expr1toexpr3and you need to make sure they get prioritized too.I don’t know if that was related to the problems you were having, but it was the cause of my own state inflation issues with question.
FlashHater
I feel so… inadequate. I need to go learn ragel and state machines, be back in a few months.
meekish
I have no experience with lexical parsers/analysers, state machines or the like. I’m just a humble (intermediate) Rubyist. But, I would like to learn more about them (i.e. become able to wrap my head around parse.y). Can you recommend a good book to get me started on the right path?
jvoorhis
Two books which helped me were the infamous Dragon Book and the O’Reilly Lex & Yacc book. Studies in Theory of Computation would also prove useful.