vendored parslet, deemed stable enough and better without dependency

lib/parslet/export.rb

@@ -0,0 +1,162 @@
+# Allows exporting parslet grammars to other lingos. 
+
+require 'set'
+require 'parslet/atoms/visitor'
+
+class Parslet::Parser
+  module Visitors
+    class Citrus
+      attr_reader :context, :output
+      def initialize(context)
+        @context = context
+      end
+      
+      def visit_str(str)
+        "\"#{str.inspect[1..-2]}\""
+      end
+      def visit_re(match)
+        match.to_s
+      end
+
+      def visit_entity(name, block)
+        context.deferred(name, block)
+
+        "(#{context.mangle_name(name)})"
+      end
+      def visit_named(name, parslet)
+        parslet.accept(self)
+      end
+
+      def visit_sequence(parslets)
+        '(' <<
+        parslets.
+          map { |el| el.accept(self) }.
+          join(' ') <<
+        ')'
+      end
+      def visit_repetition(tag, min, max, parslet)
+        parslet.accept(self) << "#{min}*#{max}"
+      end
+      def visit_alternative(alternatives)
+        '(' <<
+        alternatives.
+          map { |el| el.accept(self) }.
+          join(' | ') <<
+        ')'
+      end
+
+      def visit_lookahead(positive, bound_parslet)
+        (positive ? '&' : '!') <<
+        bound_parslet.accept(self)
+      end
+    end
+
+    class Treetop < Citrus
+      def visit_repetition(tag, min, max, parslet)
+        parslet.accept(self) << "#{min}..#{max}"
+      end
+
+      def visit_alternative(alternatives)
+        '(' <<
+        alternatives.
+          map { |el| el.accept(self) }.
+          join(' / ') <<
+        ')'
+      end
+    end
+  end
+
+  # A helper class that formats Citrus and Treetop grammars as a string. 
+  #
+  class PrettyPrinter
+    attr_reader :visitor
+    def initialize(visitor_klass)
+      @visitor = visitor_klass.new(self)
+    end
+
+    # Pretty prints the given parslet using the visitor that has been
+    # configured in initialize. Returns the string representation of the
+    # Citrus or Treetop grammar.
+    #
+    def pretty_print(name, parslet)
+      output = "grammar #{name}\n"
+      
+      output << rule('root', parslet)
+      
+      seen = Set.new
+      loop do
+        # @todo is constantly filled by the visitor (see #deferred). We 
+        # keep going until it is empty.
+        break if @todo.empty?
+        name, block = @todo.shift
+
+        # Track what rules we've already seen. This breaks loops.
+        next if seen.include?(name)
+        seen << name
+
+        output << rule(name, block.call)
+      end
+      
+      output << "end\n"
+    end
+    
+    # Formats a rule in either dialect. 
+    #
+    def rule(name, parslet)
+      "  rule #{mangle_name name}\n" << 
+      "    " << parslet.accept(visitor) << "\n" <<
+      "  end\n"
+    end
+    
+    # Whenever the visitor encounters an rule in a parslet, it defers the
+    # pretty printing of the rule by calling this method. 
+    #
+    def deferred(name, content)
+      @todo ||= []
+      @todo << [name, content]
+    end
+
+    # Mangles names so that Citrus and Treetop can live with it. This mostly
+    # transforms some of the things that Ruby allows into other patterns. If
+    # there is collision, we will not detect it for now. 
+    #
+    def mangle_name(str)
+      str.to_s.sub(/\?$/, '_p')
+    end
+  end
+
+  # Exports the current parser instance as a string in the Citrus dialect. 
+  #
+  # Example: 
+  #
+  #   require 'parslet/export'
+  #   class MyParser < Parslet::Parser
+  #     root(:expression)
+  #     rule(:expression) { str('foo') }
+  #   end
+  #   
+  #   MyParser.new.to_citrus # => a citrus grammar as a string
+  #
+  def to_citrus
+    PrettyPrinter.new(Visitors::Citrus).
+      pretty_print(self.class.name, root)
+  end
+
+  # Exports the current parser instance as a string in the Treetop dialect. 
+  #
+  # Example: 
+  #
+  #   require 'parslet/export'
+  #   class MyParser < Parslet::Parser
+  #     root(:expression)
+  #     rule(:expression) { str('foo') }
+  #   end
+  #   
+  #   MyParser.new.to_treetop # => a treetop grammar as a string
+  #
+  def to_treetop
+    PrettyPrinter.new(Visitors::Treetop).
+      pretty_print(self.class.name, root)
+  end
+end
+