Visualization of Ruby's Grammar [LWN.net]

Visualization of Ruby's Grammar

Posted Oct 31, 2006 16:47 UTC (Tue) by mikov (subscriber, #33179)
In reply to: Visualization of Ruby's Grammar by tjc
Parent article: Visualization of Ruby's Grammar

Are you saying that Ruby can not be parsed with an ANTLR grammar, or that it's just the automatic conversion from LALR(1) that is problematic? Even if the generated grammar is incorrect, I suspect that a corrected version would look substantially the same, assuming that it can be generated at all.

I have a problem with the automatic conversion. I am sure pretty much anything (well, within limits) can eventually be parsed with ANTLR using backtracking, with varying amount of effort. In this case, since the generated grammar is almost certainly not correct, and since the person who did it even neglected to mention that, it cannot be taken seriously. I have no idea whether the diagram looks similar to one for the correct grammar. Do you ? Oh, it will probably have similar node names, but that doesn't really tell us much, which is my next point.

I have no idea what the diagram really means. Where is it defined ? Who uses it ? Subjective observations that one "looks cleaner" than the other are meaningless. I could rearrange any diagram to look terrible. Perhaps one could derive some value from it, of one already had intimate knowledge about the grammar, but not the other way around.

On the contrary, I find this very interesting! See Bruce Person's comment about the primary node in Ruby, for example.

I agree that his comment is interesting. Then I am pretty sure it is because he probably knows Ruby, not because he actually used the diagram to derive insight. His comments about the size of the parser make sense in the same context. The observation about the lexical analyzer doing a lot of work especially doesn't stand on its own - it is impossible to deduce that from the diagram.

The problem is that the entire thing it is based on a wrong premise. Even if it can generate interesting comments and useful tidbits of information, ultimately it is misleading. A casual reader could be lead to believe that:

Conversion between LR and LL is trivial.
This diagram is the true diagram of Ruby's syntax. (whatever that means)
Diagrams are often used as tools to study and evaluate grammars
Subjective evaluation of the artistic properties of the diagram corresponds to properties of the language.

All these things are false.

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