Merge pull request #157 from chrismwendt/ignore

Add the ability to ignore a parse result

lib/parslet/atoms.rb

@@ -19,6 +19,7 @@ module Precedence
   require 'parslet/atoms/context'
   require 'parslet/atoms/dsl'
   require 'parslet/atoms/base'
+  require 'parslet/atoms/ignored'
   require 'parslet/atoms/named'
   require 'parslet/atoms/lookahead'
   require 'parslet/atoms/alternative'

lib/parslet/atoms/can_flatten.rb

@@ -120,7 +120,7 @@ def flatten_repetition(list, named)
       return [] if named && list.empty?
 
       # If there are only strings, concatenate them and return that. 
-      foldl(list) { |s,e| s+e }
+      foldl(list.compact) { |s,e| s+e }
     end
 
     # That annoying warning 'Duplicate subtrees while merging result' comes 

lib/parslet/atoms/dsl.rb

@@ -35,6 +35,17 @@ def maybe
     Parslet::Atoms::Repetition.new(self, 0, 1, :maybe)
   end
 
+  # Returns a new parslet atom that will not show up in the output. This
+  # is synonymous to calling #repeat(0,1). Generated tree value will always be 
+  # nil.
+  #
+  # Example: 
+  #   str('foo').ignore
+  #
+  def ignore
+    Parslet::Atoms::Ignored.new(self)
+  end
+
   # Chains two parslet atoms together as a sequence. 
   #
   # Example: 

lib/parslet/atoms/ignored.rb

@@ -0,0 +1,26 @@
+# Ignores the result of a match.
+#
+# Example:
+#
+#   str('foo')            # will return 'foo',
+#   str('foo').ignore     # will return nil
+#
+class Parslet::Atoms::Ignored < Parslet::Atoms::Base
+  attr_reader :parslet
+  def initialize(parslet)
+    super()
+
+    @parslet = parslet
+  end
+
+  def apply(source, context, consume_all)
+    success, _ = result = parslet.apply(source, context, consume_all)
+
+    return result unless success
+    succ(nil)
+  end
+
+  def to_s_inner(prec)
+    "ignored(#{parslet.to_s(prec)})"
+  end
+end

spec/parslet/atoms/ignored_spec.rb

@@ -0,0 +1,15 @@
+require 'spec_helper'
+
+describe Parslet::Atoms::Ignored do
+  include Parslet
+
+  describe "ignore" do
+    it "ignores parts of the input" do
+      str('a').ignore.parse('a').should == nil
+      (str('a') >> str('b').ignore >> str('c')).parse('abc').should == 'ac'
+      (str('a') >> str('b').as(:name).ignore >> str('c')).parse('abc').should == 'ac'
+      (str('a') >> str('b').maybe.ignore >> str('c')).parse('abc').should == 'ac'
+      (str('a') >> str('b').maybe.ignore >> str('c')).parse('ac').should == 'ac'
+    end
+  end
+end

website/source/index.html.textile

@@ -11,13 +11,13 @@ title: About
   # Constructs a parser using a Parser Expression Grammar 
   parser =  str('"') >> 
             (
-              str('\\') >> any |
+              str('\\').ignore >> any |
               str('"').absent? >> any
             ).repeat.as(:string) >> 
             str('"')
 
-  result = parser.parse %Q("this is a valid string") 
-  result # => {:string=>"this is a valid string"@1}
+  result = parser.parse %Q("this is a valid \\"string\\"")
+  result # => {:string=>"this is a valid \"string\""@1}
 </code></pre>
 
 A small Ruby library for constructing parsers in the