ruby -y

For people who are interested in how ruby's yacc parser works, there is an undocumented command line option("-y") that may be helpful. It will display a trace of the parser's operations.

To use it, you need to clear your RUBYOPT environment variable to NOT use "rubygems" (this will break some ruby applications), otherwise it will make too much noise.

Here is an example:

$ruby -y -e "a=1"
Starting parse
Entering state 0
Reducing stack by rule 1 (line 328), -> @1
Stack now 0
Entering state 2
Reading a token: Next token is token tIDENTIFIER ()
Shifting token tIDENTIFIER, Entering state 34
Reading a token: Next token is token '=' ()
Reducing stack by rule 418 (line 2146), tIDENTIFIER -> variable
Stack now 0 2
Entering state 90
Next token is token '=' ()
Reducing stack by rule 83 (line 827), variable -> lhs
Stack now 0 2
Entering state 73
Next token is token '=' ()
Shifting token '=', Entering state 315
Reading a token: Next token is token tINTEGER ()
Shifting token tINTEGER, Entering state 40
Reducing stack by rule 414 (line 2134), tINTEGER -> numeric
Stack now 0 2 73 315
Entering state 89
Reducing stack by rule 376 (line 1899), numeric -> literal
Stack now 0 2 73 315
Entering state 79
Reducing stack by rule 267 (line 1421), literal -> primary
Stack now 0 2 73 315
Entering state 75
Reading a token: Next token is token '\n' ()
Reducing stack by rule 217 (line 1199), primary -> arg
Stack now 0 2 73 315
Entering state 488
Next token is token '\n' ()
Reducing stack by rule 173 (line 953), lhs '=' arg -> arg
Stack now 0 2
Entering state 74
Next token is token '\n' ()
Reducing stack by rule 40 (line 616), arg -> expr
Stack now 0 2
Entering state 64
Next token is token '\n' ()
Reducing stack by rule 34 (line 596), expr -> stmt
Stack now 0 2
Entering state 63
Next token is token '\n' ()
Reducing stack by rule 6 (line 381), stmt -> stmts
Stack now 0 2
Entering state 62
Next token is token '\n' ()
Shifting token '\n', Entering state 216
Reducing stack by rule 496 (line 2429), '\n' -> term
Stack now 0 2 62
Entering state 220
Reducing stack by rule 497 (line 2432), term -> terms
Stack now 0 2 62
Entering state 300
Reading a token: Now at end of input.
Reducing stack by rule 489 (line 2416), terms -> opt_terms
Stack now 0 2 62
Entering state 299
Reducing stack by rule 4 (line 373), stmts opt_terms -> compstmt
Stack now 0 2
Entering state 61
Reducing stack by rule 2 (line 328), @1 compstmt -> program
Stack now 0
Entering state 1
Now at end of input.

XRuby's ANTLR parser does not have this option. But as ANRLE produces human readable code, you can just read the generated code or step through it in a debugger to learn how it works.