add Slop::Result#fetch

use Slop::Result#clean_key in Slop::Result#option

fix typo in Slop::Result#[] and Slop::Result#fetch descriptions

handle case when Slop::Result#fetch tries to fetch an option that is not provided and does not have a default value

raise Slop::UnknownOption when Slop::Result#fetch tries to fetch an unexisting key

set Slop::Result#clean_key method as private

remove redundant Slop::Result#fetch tests

update description of Slop::Result#fetch test when trying to access an unexisting option

update error message when an option is not present on Slop::Result#fetch

description of Slop::Result#fetch

update expected error message on test for Slop::Result#fetch when an option is not present

3 files changed, 49 insertions, 9 deletions

diff --git a/lib/slop/error.rb b/lib/slop/error.rb
index e024645..ddfa1a7 100644
--- a/lib/slop/error.rb
+++ b/lib/slop/error.rb
@@ -22,8 +22,9 @@ module Slop
     end
   end
 
-  # Raised when an unknown option is parsed. Suppress
-  # with the `suppress_errors` config option.
+  # Raised when an unknown option is parsed or when trying to fetch an
+  # unexisting option via `Slop::Result#fetch`.
+  # Suppress with the `suppress_errors` config option.
   class UnknownOption < Error
     attr_reader :flag
 
diff --git a/lib/slop/result.rb b/lib/slop/result.rb
index 469d05c..0be3c11 100644
--- a/lib/slop/result.rb
+++ b/lib/slop/result.rb
@@ -14,12 +14,23 @@ module Slop
       @options = parser.options
     end
 
-    # Returns an options value, nil if the option does not exist.
+    # Returns an option's value, nil if the option does not exist.
     def [](flag)
       (o = option(flag)) && o.value
     end
     alias get []
 
+    # Returns an option's value, raises UnknownOption if the option does not exist.
+    def fetch(flag)
+      o = option(flag)
+      if o.nil?
+        cleaned_key = clean_key(flag)
+        raise UnknownOption.new("option not found: '#{cleaned_key}'", "#{cleaned_key}")
+      else
+        o.value
+      end
+    end
+
     # Set the value for an option. Raises an ArgumentError if the option
     # does not exist.
     def []=(flag, value)
@@ -33,13 +44,8 @@ module Slop
 
     # Returns an Option if it exists. Ignores any prefixed hyphens.
     def option(flag)
-      cleaned = -> (f) do
-        key = f.to_s.sub(/\A--?/, '')
-        key = key.tr '-', '_' if parser.config[:underscore_flags]
-        key.to_sym
-      end
       options.find do |o|
-        o.flags.any? { |f| cleaned.(f) == cleaned.(flag) }
+        o.flags.any? { |f| clean_key(f) == clean_key(flag) }
       end
     end
 
@@ -90,5 +96,13 @@ module Slop
     def to_s(**opts)
       options.to_s(**opts)
     end
+
+    private
+
+    def clean_key(key)
+      key = key.to_s.sub(/\A--?/, '')
+      key = key.tr '-', '_' if parser.config[:underscore_flags]
+      key.to_sym
+    end
   end
 end
diff --git a/test/result_test.rb b/test/result_test.rb
index 322daa1..30e4934 100644
--- a/test/result_test.rb
+++ b/test/result_test.rb
@@ -62,6 +62,31 @@ describe Slop::Result do
     end
   end
 
+  describe "#fetch" do
+    it "returns an options value" do
+      assert_equal "lee", @result.fetch("--name")
+    end
+
+    it "raises Slop::UnknownOption when an option does not exists" do
+      e = assert_raises(Slop::UnknownOption) { @result.fetch("--unexisting") }
+      assert_equal "option not found: 'unexisting'", e.message
+    end
+
+    it "returns the default value of an option when a value is not provided" do
+      @options.string("--foo", default: "bar")
+      @result.parser.parse %w(--foo)
+
+      assert_equal 'bar', @result.fetch('foo')
+    end
+
+    it "returns nil when an option is not provided and it does not have a default value" do
+      @options.string("--hello")
+      @result.parser.parse %w()
+
+      assert_equal nil, @result.fetch('hello')
+    end
+  end
+
   describe "#[]=" do
     it "sets an options value" do
       assert_equal "lee", @result["name"]