Support for required options

This commit introduces support for required options,
which are options that cause the parser to raise
a `MissingRequiredOption` exception if not present.

Options can be marked as required by passing
`required: true` in their configuration, and any errors
caused by missing required options can be suppressed via
`suppress_errors: true`.

3 files changed, 20 insertions, 0 deletions

diff --git a/lib/slop/error.rb b/lib/slop/error.rb
index b02de3b..e024645 100644
--- a/lib/slop/error.rb
+++ b/lib/slop/error.rb
@@ -32,4 +32,9 @@ module Slop
       @flag = flag
     end
   end
+
+  # Raised when a required option is *not* parsed.
+  # Suppress with the `suppress_errors` config option.
+  class MissingRequiredOption < Error
+  end
 end
diff --git a/lib/slop/option.rb b/lib/slop/option.rb
index b8533ae..093b37f 100644
--- a/lib/slop/option.rb
+++ b/lib/slop/option.rb
@@ -4,6 +4,7 @@ module Slop
       help: true,
       tail: false,
       underscore_flags: true,
+      required: false,
     }
 
     # An Array of flags this option matches.
@@ -101,6 +102,11 @@ module Slop
       config[:suppress_errors]
     end
 
+    # Returns true if an exception should be raised when this option isn't supplied.
+    def required?
+      config[:required]
+    end
+
     # Returns all flags joined by a comma. Used by the help string.
     def flag
       flags.join(", ")
diff --git a/lib/slop/parser.rb b/lib/slop/parser.rb
index 3cba82b..463e75a 100644
--- a/lib/slop/parser.rb
+++ b/lib/slop/parser.rb
@@ -80,6 +80,15 @@ module Slop
 
       @arguments += ignored_args
 
+      if !suppress_errors?
+        unused_options.each do |o|
+          if o.config[:required]
+            pretty_flags = o.flags.map { |f| "`#{f}'" }.join(", ")
+            raise MissingRequiredOption, "missing required option #{pretty_flags}"
+          end
+        end
+      end
+
       Result.new(self).tap do |result|
         used_options.each { |o| o.finish(result) }
       end