- Add 'required' option support:

    option "--foo", "BAR", "Some description", :required => true

  Notes:
    - :required and :default are mutually exclusive as a sanity check.
    - :required conflicts with :flag options since that doesn't make
      sense either.

lib/clamp/option.rb

@@ -17,6 +17,16 @@ def initialize(switches, type, description, options = {})
       if options.has_key?(:environment_variable)
         @environment_variable = options[:environment_variable]
       end
+      if options.has_key?(:required)
+        @required = options[:required]
+        # Do some light validation for conflicting settings.
+        if options.has_key?(:default)
+          raise ArgumentError, "Specifying a :default value also :required doesn't make sense"
+        end
+        if type == :flag
+          raise ArgumentError, "A required flag (boolean) doesn't make sense."
+        end
+      end
     end
 
     attr_reader :switches, :type
@@ -33,6 +43,10 @@ def handles?(switch)
       recognised_switches.member?(switch)
     end
 
+    def required?
+      @required
+    end
+
     def flag?
       @type == :flag
     end

lib/clamp/option/parsing.rb

@@ -52,6 +52,19 @@ def parse_options
           end
 
         end
+
+        # Verify that all required options are present
+        self.class.recognised_options.each do |option|
+          # If this option is required and the value is nil, there's an error.
+          if option.required? and send(option.attribute_name).nil?
+            message = "option '#{option.switches.first}'"
+            if option.environment_variable
+              message += " (or env #{option.environment_variable})"
+            end
+            message += " is required"
+            signal_usage_error message
+          end
+        end
       end
 
       def parse_environment_options