Rockit: A Parser Generator for Ruby

Rockit Jason Morrison Language Processors and Compiler Construction Winter 2006-3

Ruby Rockit: Overview
It used to be GLR.
Maybe in the future it will be PEG.
It’s kind of there.
Let’s see!

It used to be GLR
First home I found:
http://rockit.sourceforge.net/
Version 0.3.8, updated October 2001
What did it look like?

It used to be GLR.
Let’s see!

It used to be GLR.
What’s that?
Let’s see!

What’s GLR?
Generalized LR
Parse tables have multiple transitions
When it hits S/R or R/R conflict:
Fork parse stack
Follow both paths
Got a rule that produces no transition? Discard that stack.

How well does it work?
Run time scales with degree of nondeterminism.
O(n) on deterministic grammar.
Optimize: share common stack prefix/suffix. (Leads to a state lattice rather than stack)

It used to be GLR.
Let’s see!

Rockit 0.8.0
C-based parser, Ruby production rules
Unambiguous,  lookahead, O(n) Packrat parsing
Ruby and Java grammars, analyzers, pretty-printers

It used to be GLR.
… PEG? Packrat?
Let’s see!

What’s PEG?
“Parsing Expression Grammar”
No separate tokenizing phase
Unambiguous (one tree for valid parse)
No left recursion (as with LL)

What’s PEG?
Fixes “dangling else” via prioritization
Infinite lookahead via syntactic predicates
Let’s hear it from B ryan Ford (coined PEG)
http://www.bford.info/pub/lang/peg-slides.pdf

What’s Packrat?
Essentially: recursive descent
Memoization yields O(n) with unlimited lookahead and backtracking
Packrat parsers:
Java: Rats! (from xtc, eXTensible C project)
Haskell: Pappy

Packrat: Since when?
Theory has been formalized since the 70s
Memoization was too heavy, then.
Now, RAM is cheap; memoization often consumes less than subsequent compilation steps.

It used to be GLR.
Let’s see!

It used to be GLR.
What do you mean by that?
Let’s see!

Rockit 0.7.2
Newest available
Deemed 0.8.0 preview
Pure Ruby (=> slow) recursive-descent parser
Does not support syntactic predicates
Examples are only documentation
Unorganized source
No error hooks

It used to be GLR.
Let’s see!
Let’s see it anyways!

Rockit 0.7.2
Arithmetic expression example

Conclusion
0.7.2 is interesting, but doesn’t have:
Syntactic predicates
Memoization
Still denotes grammar in code.
Pros: Quick, familiar.
Cons: Difficult to analyze.
If 0.8.0 is released as promised, should be great!

Resources
These slides and I:
http:// jayunit.net/rockit
jason.p.morrison@gmail.com
PEG ML:
https:// lists.csail.mit.edu/mailman/listinfo/peg
Bryan Ford:
http:// pdos.csail.mit.edu/~baford/packrat
Wikipedia, Google for:
GLR parser, PEG grammar, Packrat parser

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Rockit: A Parser Generator for Ruby

by Jason Morrison on Dec 27, 2007

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Rockit: A Parser Generator for Ruby — Presentation Transcript