Support --no- prefix for inverting boolean options

closes #168

README.md

@@ -72,10 +72,10 @@ opts.int "-p", "--port", "a port", default: 80
 opts.separator ""
 opts.separator "Extra options:"
 opts.array "--files", "a list of files to import"
-opts.bool "-v", "--verbose", "enable verbose mode"
+opts.bool "-v", "--verbose", "enable verbose mode", default: true
 
 parser = Slop::Parser.new(opts)
-result = parser.parse(["--hostname", "192.168.0.1"])
+result = parser.parse(["--hostname", "192.168.0.1", "--no-verbose"])
 
 result.to_hash #=> { hostname: "192.168.0.1", port: 80,
                  #     files: [], verbose: false }

lib/slop/parser.rb

@@ -92,6 +92,8 @@ def process(option, arg)
     def try_process(flag, arg)
       if option = matching_option(flag)
         process(option, arg)
+      elsif flag.start_with?("--no-") && option = matching_option(flag.sub("no-", ""))
+        process(option, false)
       elsif flag =~ /\A-[^-]/ && flag.size > 2
         # try and process as a set of grouped short flags. drop(1) removes
         # the prefixed -, then we add them back to each flag separately.

lib/slop/types.rb

@@ -8,12 +8,29 @@ def call(value)
 
   # Cast the option argument to true or false.
   # Override default_value to default to false instead of nil.
-  # This option type does not expect an argument.
+  # This option type does not expect an argument. However, the API
+  # supports value being passed. This is to ensure it can capture
+  # an explicit false value
   class BoolOption < Option
-    def call(_value)
+    attr_accessor :explicit_value
+
+    def call(value)
+      self.explicit_value = value
       true
     end
 
+    def value
+      if force_false?
+        false
+      else
+        super
+      end
+    end
+
+    def force_false?
+      explicit_value == false
+    end
+
     def default_value
       config[:default] || false
     end

test/types_test.rb

@@ -2,10 +2,11 @@
 
 describe Slop::BoolOption do
   before do
-    @options = Slop::Options.new
-    @age     = @options.bool "--verbose"
-    @age     = @options.bool "--quiet"
-    @result  = @options.parse %w(--verbose)
+    @options  = Slop::Options.new
+    @verbose  = @options.bool "--verbose"
+    @quiet    = @options.bool "--quiet"
+    @inversed = @options.bool "--inversed", default: true
+    @result   = @options.parse %w(--verbose --no-inversed)
   end
 
   it "returns true if used" do
@@ -15,6 +16,10 @@
   it "returns false if not used" do
     assert_equal false, @result[:quiet]
   end
+
+  it "can be inversed via --no- prefix" do
+    assert_equal false, @result[:inversed]
+  end
 end
 
 describe Slop::IntegerOption do