Add NullOption and default to using it

A NullOption is one whos return value we don't care about. For example,
you might just want a `--version` option which simply prints the
version and exits. In this case, having a `true` value in `to_hash` is
really just noise. We probably don't care about it, using NullOption
discards it.

I think using this for Options#on makes sense because this is logical:

    opts.on '--version' do
      puts VERSION
      exit
    end

Rather than:

    opts.add '--version' do
      puts VERSION
      exit
    end

And defaulting to a StringOption. This also means you have to be
explicit about adding such an option, which is a good thing.

2 files changed, 28 insertions, 13 deletions

diff --git a/test/options_test.rb b/test/options_test.rb
index 57083ac..5b12f7b 100644
--- a/test/options_test.rb
+++ b/test/options_test.rb
@@ -5,34 +5,34 @@ describe Slop::Options do
     @options = Slop::Options.new
   end
 
-  describe "#add" do
-    it "defaults to string type" do
-      assert_kind_of Slop::StringOption, @options.add("--foo")
+  describe "#on" do
+    it "defaults to null type" do
+      assert_kind_of Slop::NullOption, @options.on("--foo")
     end
 
     it "accepts custom types" do
       module Slop; class FooOption < Option; end; end
-      assert_kind_of Slop::FooOption, @options.add("--foo", type: :foo)
+      assert_kind_of Slop::FooOption, @options.on("--foo", type: :foo)
     end
 
     it "adds multiple flags" do
-      option = @options.add("-f", "-F", "--foo")
+      option = @options.on("-f", "-F", "--foo")
       assert_equal %w(-f -F --foo), option.flags
     end
 
     it "accepts a trailing description" do
-      option = @options.add("--foo", "fooey")
+      option = @options.on("--foo", "fooey")
       assert_equal "fooey", option.desc
     end
 
     it "adds the option" do
-      option = @options.add("--foo")
+      option = @options.on("--foo")
       assert_equal [option], @options.to_a
     end
 
     it "raises an error when a duplicate flag is used" do
-      @options.add("--foo")
-      assert_raises(ArgumentError) { @options.add("--foo") }
+      @options.on("--foo")
+      assert_raises(ArgumentError) { @options.on("--foo") }
     end
   end
 
@@ -60,19 +60,19 @@ describe Slop::Options do
     end
 
     it "aligns option strings" do
-      @options.add "-f", "--foo", "fooey"
-      @options.add "-s", "short"
+      @options.on "-f", "--foo", "fooey"
+      @options.on "-s", "short"
       assert_match(/^    -f, --foo  fooey/, @options.to_s)
       assert_match(/^    -s         short/, @options.to_s)
     end
 
     it "can use a custom prefix" do
-      @options.add "-f", "--foo"
+      @options.on "-f", "--foo"
       assert_match(/^ -f, --foo/, @options.to_s(prefix: " "))
     end
 
     it "ignores options with help: false" do
-      @options.add "-x", "something", help: false
+      @options.on "-x", "something", help: false
       refute_match(/something/, @options.to_s)
     end
   end
diff --git a/test/types_test.rb b/test/types_test.rb
index 44846b1..3696126 100644
--- a/test/types_test.rb
+++ b/test/types_test.rb
@@ -61,3 +61,18 @@ describe Slop::ArrayOption do
   end
 end
 
+describe Slop::NullOption do
+  before do
+    @options = Slop::Options.new
+    @version = @options.null('--version')
+    @result  = @options.parse %w(--version)
+  end
+
+  it 'has a return value of true' do
+    assert_equal true, @result[:version]
+  end
+
+  it 'is not included in to_hash' do
+    assert_equal({}, @result.to_hash)
+  end
+end