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.

4 files changed, 37 insertions, 4 deletions

diff --git a/lib/slop/option.rb b/lib/slop/option.rb
index 089d36b..85d7cb7 100644
--- a/lib/slop/option.rb
+++ b/lib/slop/option.rb
@@ -53,6 +53,12 @@ module Slop
       true
     end
 
+    # Override this if you want to ignore the return value for an option
+    # (i.e so Result#to_hash does not include it).
+    def null?
+      false
+    end
+
     def value
       @value || default_value
     end
diff --git a/lib/slop/options.rb b/lib/slop/options.rb
index f89e857..34ebe86 100644
--- a/lib/slop/options.rb
+++ b/lib/slop/options.rb
@@ -4,7 +4,7 @@ module Slop
 
     DEFAULT_CONFIG = {
       suppress_errors: false,
-      type:            "string",
+      type:            "null",
     }
 
     attr_reader :options
@@ -23,7 +23,21 @@ module Slop
       yield self if block_given?
     end
 
-    def add(*flags, **config, &block)
+    # Add a new option. This method is an alias for adding a NullOption
+    # (i.e an option with an ignored return value).
+    #
+    # Example:
+    #
+    #   opts = Slop.parse do |o|
+    #     o.on '--version' do
+    #       puts Slop::VERSION
+    #     end
+    #   end
+    #
+    #   opts.to_hash #=> {}
+    #
+    # Returns the newly created Option subclass.
+    def on(*flags, **config, &block)
       desc   = flags.pop unless flags.last.start_with?('-')
       config = self.config.merge(config)
       klass  = Slop.string_to_option_class(config[:type].to_s)
@@ -32,6 +46,8 @@ module Slop
       add_option option
     end
 
+    # Add a separator between options. Used when displaying
+    # the help text.
     def separator(string)
       if separators[options.size]
         separators.last << "\n#{string}"
@@ -40,10 +56,12 @@ module Slop
       end
     end
 
+    # Sugar to avoid `options.parser.parse(x)`.
     def parse(strings)
       parser.parse(strings)
     end
 
+    # Implements the Enumerable interface.
     def each(&block)
       options.each(&block)
     end
@@ -51,7 +69,7 @@ module Slop
     def method_missing(name, *args, **config, &block)
       if respond_to_missing?(name)
         config[:type] = name
-        add(*args, config, &block)
+        on(*args, config, &block)
       else
         super
       end
@@ -65,6 +83,7 @@ module Slop
       options.dup
     end
 
+    # Returns the help text for this options. Used by Result#to_s.
     def to_s(prefix: " " * 4)
       str = banner + "\n"
       len = longest_flag_length
diff --git a/lib/slop/result.rb b/lib/slop/result.rb
index 23d023b..03b369b 100644
--- a/lib/slop/result.rb
+++ b/lib/slop/result.rb
@@ -50,7 +50,7 @@ module Slop
 
     # Returns a hash with option key => value.
     def to_hash
-      Hash[options.map { |o| [o.key, o.value] }]
+      Hash[options.reject(&:null?).map { |o| [o.key, o.value] }]
     end
     alias to_h to_hash
 
diff --git a/lib/slop/types.rb b/lib/slop/types.rb
index 8576421..c7db245 100644
--- a/lib/slop/types.rb
+++ b/lib/slop/types.rb
@@ -41,4 +41,12 @@ module Slop
       config[:delimiter] || ","
     end
   end
+
+  # an option that discards the return value
+  class NullOption < BoolOption
+    def null?
+      true
+    end
+  end
+
 end