[PoC] Intoroduce parameterizing rules with conditonal

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.

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`.

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](https://github.com/ruby/ruby/blob/2f916812a9b818b432ee7c299e021ec62d4727fb/parse.y#L5523-L5575) > [args_tail](https://github.com/ruby/ruby/blob/2f916812a9b818b432ee7c299e021ec62d4727fb/parse.y#L5487-L5503) > [args_forward](https://github.com/ruby/ruby/blob/2f916812a9b818b432ee7c299e021ec62d4727fb/parse.y#L5586-L5597), 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.

Author: ydah

lib/lrama/grammar.rb

@@ -30,7 +30,7 @@ class Grammar
                   :after_shift, :before_reduce, :after_reduce, :after_shift_error_token, :after_pop_stack,
                   :symbols_resolver, :types,
                   :rules, :rule_builders,
-                  :sym_to_rules, :no_stdlib
+                  :sym_to_rules, :no_stdlib, :if_count
 
     def_delegators "@symbols_resolver", :symbols, :nterms, :terms, :add_nterm, :add_term,
                                         :find_symbol_by_number!, :find_symbol_by_id!, :token_to_symbol,
@@ -58,6 +58,7 @@ def initialize(rule_counter)
       @accept_symbol = nil
       @aux = Auxiliary.new
       @no_stdlib = false
+      @if_count = 0
 
       append_special_symbols
     end
@@ -170,6 +171,10 @@ def find_rules_by_symbol(sym)
       @sym_to_rules[sym.number]
     end
 
+    def initialize_if_count
+      @if_count = 0
+    end
+
     private
 
     def compute_nullable

lib/lrama/grammar/binding.rb

@@ -15,6 +15,10 @@ def resolve_symbol(symbol)
         if symbol.is_a?(Lexer::Token::InstantiateRule)
           resolved_args = symbol.args.map { |arg| resolve_symbol(arg) }
           Lrama::Lexer::Token::InstantiateRule.new(s_value: symbol.s_value, location: symbol.location, args: resolved_args, lhs_tag: symbol.lhs_tag)
+        elsif symbol.is_a?(Lexer::Token::ControlSyntax)
+          resolved = symbol.dup
+          resolved.condition = @parameter_to_arg[symbol.condition_value]
+          resolved
         else
           @parameter_to_arg[symbol.s_value] || symbol
         end

lib/lrama/grammar/parameterizing_rule/rhs.rb

@@ -10,6 +10,31 @@ def initialize
           @precedence_sym = nil
         end
 
+        def skip?(bindings)
+          return false unless @symbols.last
+          last_sym = bindings.resolve_symbol(@symbols.last)
+          last_sym.is_a?(Lexer::Token::ControlSyntax) && last_sym.if? && last_sym.false?
+        end
+
+        def resolve_symbols(bindings)
+          is_skip = []
+          @symbols.map do |sym|
+            resolved = bindings.resolve_symbol(sym)
+            if resolved.is_a?(Lexer::Token::ControlSyntax)
+              if resolved.if?
+                is_skip.push(resolved.false?)
+              elsif resolved.endif?
+                is_skip.pop
+              else
+                raise "Unexpected control syntax: #{resolved.condition_value}"
+              end
+              nil
+            else
+              resolved unless is_skip.last
+            end
+          end.compact
+        end
+
         def resolve_user_code(bindings)
           return unless user_code
 

lib/lrama/grammar/rule_builder.rb

@@ -127,13 +127,17 @@ def process_rhs
             if (created_lhs = @parameterizing_rule_resolver.created_lhs(lhs_s_value))
               @replaced_rhs << created_lhs
             else
+              next if parameterizing_rule.rhs_list.all? { |r| r.skip?(bindings) }
               lhs_token = Lrama::Lexer::Token::Ident.new(s_value: lhs_s_value, location: token.location)
               @replaced_rhs << lhs_token
               @parameterizing_rule_resolver.created_lhs_list << lhs_token
               parameterizing_rule.rhs_list.each do |r|
                 rule_builder = RuleBuilder.new(@rule_counter, @midrule_action_counter, @parameterizing_rule_resolver, lhs_tag: token.lhs_tag || parameterizing_rule.tag)
                 rule_builder.lhs = lhs_token
-                r.symbols.each { |sym| rule_builder.add_rhs(bindings.resolve_symbol(sym)) }
+                next if r.skip?(bindings)
+                r.resolve_symbols(bindings).each do |sym|
+                  rule_builder.add_rhs(sym)
+                end
                 rule_builder.line = line
                 rule_builder.precedence_sym = r.precedence_sym
                 rule_builder.user_code = r.resolve_user_code(bindings)

lib/lrama/lexer.rb

@@ -38,6 +38,10 @@ class Lexer
       %rule
       %no-stdlib
       %inline
+      %if
+      %endif
+      %true
+      %false
     )
 
     def initialize(grammar_file)

lib/lrama/lexer/token.rb

@@ -1,4 +1,5 @@
 require 'lrama/lexer/token/char'
+require 'lrama/lexer/token/control_syntax'
 require 'lrama/lexer/token/ident'
 require 'lrama/lexer/token/instantiate_rule'
 require 'lrama/lexer/token/tag'

lib/lrama/lexer/token/control_syntax.rb

@@ -0,0 +1,34 @@
+module Lrama
+  class Lexer
+    class Token
+      class ControlSyntax < Token
+        attr_accessor :condition
+
+        def initialize(s_value:, location:, condition: nil)
+          @condition = condition
+          super(s_value: s_value, location: location)
+        end
+
+        def if?
+          s_value == '%if'
+        end
+
+        def endif?
+          s_value == '%endif'
+        end
+
+        def true?
+          !!@condition&.s_value
+        end
+
+        def false?
+          !true?
+        end
+
+        def condition_value
+          @condition&.s_value
+        end
+      end
+    end
+  end
+end