[PoC] Intoroduce parameterizing rules with conditional statement

[ydah]

I would like to propose a new grammar in this PR.
I believe that more parameterizing rules can handle more abstract rules if we can switch between rules and actions that are expanded by conditions in order to make rules common.

DesignDoc: https://gist.github.com/ydah/62008655a6f6c3118ab01134dc91da6f

Syntax is as follows:

%rule defined_rule(X, condition): /* empty */
                                | X { $$ = $1; } %if(condition) /* 1 */
                                | %if(condition) X %endif X { $$ = $1; } /* 2 */
                                ;

%%

r_true        : defined_rule(number, %true)
              ;

r_false       : defined_rule(number, %false)
              ;

1. It's like a postfix if in Ruby. If condition is false, it is equivalent to missing this line.
2. If statementIf condition is false, it is equivalent to missing RHS between %if and% endif.

❯ exe/lrama --trace=rules spec/fixtures/parameterizing_rules/user_defined/if.y
Grammar rules:
$accept -> r_true YYEOF
defined_rule_number_true -> ε
defined_rule_number_true -> number
defined_rule_number_true -> number number
r_true -> defined_rule_number_true
defined_rule_number_false -> ε
defined_rule_number_false -> number
r_false -> defined_rule_number_false

Motivation

I believe it will solve the problem mentioned in the article below with the tight coupling with Lexer "to disable certain generation rules under certain conditions" and I would like to propose this feature to solve this problem.
https://yui-knk.hatenablog.com/entry/2023/04/04/190413

We can trace the RHS to f_args > args_tail > args_forward, where f_args is the RHS of both the lambda argument (f_larglist) and the method definition argument (f_arglist).
So if we can switch between RHS and actions by passing parameters, we can break up the Lexer/Parser coupling here.