diff --git a/lib/optparse.rb b/lib/optparse.rb
index ccfea6d..fbcd7f9 100644
--- a/lib/optparse.rb
+++ b/lib/optparse.rb
@@ -697,6 +697,11 @@ def pretty_print(q)         # :nodoc:
       q.object_group(self) {pretty_print_contents(q)}
     end
 
+    def omitted_argument(val)   # :nodoc:
+      val.pop if val.size == 3 and val.last.nil?
+      val
+    end
+
     #
     # Switch that takes no arguments.
     #
@@ -755,7 +760,7 @@ def parse(arg, argv, &error)
         if arg
           conv_arg(*parse_arg(arg, &error))
         else
-          conv_arg(arg)
+          omitted_argument conv_arg(arg)
         end
       end
 
@@ -774,13 +779,14 @@ class PlacedArgument < self
       #
       def parse(arg, argv, &error)
         if !(val = arg) and (argv.empty? or /\A-./ =~ (val = argv[0]))
-          return nil, block, nil
+          return nil, block
         end
         opt = (val = parse_arg(val, &error))[1]
         val = conv_arg(*val)
         if opt and !arg
           argv.shift
         else
+          omitted_argument val
           val[0] = nil
         end
         val
@@ -1658,9 +1664,9 @@ def parse_in_order(argv = default_argv, setter = nil, &nonopt)  # :nodoc:
             raise $!.set_option(arg, true)
           end
           begin
-            opt, cb, val = sw.parse(rest, argv) {|*exc| raise(*exc)}
-            val = cb.call(val) if cb
-            setter.call(sw.switch_name, val) if setter
+            opt, cb, *val = sw.parse(rest, argv) {|*exc| raise(*exc)}
+            val = callback!(cb, 1, *val) if cb
+            callback!(setter, 2, sw.switch_name, *val) if setter
           rescue ParseError
             raise $!.set_option(arg, rest)
           end
@@ -1690,7 +1696,7 @@ def parse_in_order(argv = default_argv, setter = nil, &nonopt)  # :nodoc:
             raise $!.set_option(arg, true)
           end
           begin
-            opt, cb, val = sw.parse(val, argv) {|*exc| raise(*exc) if eq}
+            opt, cb, *val = sw.parse(val, argv) {|*exc| raise(*exc) if eq}
           rescue ParseError
             raise $!.set_option(arg, arg.length > 2)
           else
@@ -1698,8 +1704,8 @@ def parse_in_order(argv = default_argv, setter = nil, &nonopt)  # :nodoc:
           end
           begin
             argv.unshift(opt) if opt and (!rest or (opt = opt.sub(/\A-*/, '-')) != '-')
-            val = cb.call(val) if cb
-            setter.call(sw.switch_name, val) if setter
+            val = callback!(cb, 1, *val) if cb
+            callback!(setter, 2, sw.switch_name, *val) if setter
           rescue ParseError
             raise $!.set_option(arg, arg.length > 2)
           end
@@ -1725,6 +1731,17 @@ def parse_in_order(argv = default_argv, setter = nil, &nonopt)  # :nodoc:
   end
   private :parse_in_order
 
+  # Calls callback with _val_.
+  def callback!(cb, max_arity, *args) # :nodoc:
+    if (size = args.size) < max_arity and cb.to_proc.lambda?
+      (arity = cb.arity) < 0 and arity = (1-arity)
+      arity = max_arity if arity > max_arity
+      args[arity - 1] = nil if arity > size
+    end
+    cb.call(*args)
+  end
+  private :callback!
+
   #
   # Parses command line arguments +argv+ in permutation mode and returns
   # list of non-option arguments. When optional +into+ keyword
diff --git a/test/optparse/test_acceptable.rb b/test/optparse/test_acceptable.rb
index 12f5322..c7ea215 100644
--- a/test/optparse/test_acceptable.rb
+++ b/test/optparse/test_acceptable.rb
@@ -8,6 +8,7 @@ def setup
     @opt.def_option("--integer VAL", Integer) { |v| @integer = v }
     @opt.def_option("--float VAL",   Float)   { |v| @float   = v }
     @opt.def_option("--numeric VAL", Numeric) { |v| @numeric = v }
+    @opt.def_option("--array VAL", Array) { |v| @array = v }
 
     @opt.def_option("--decimal-integer VAL",
                     OptionParser::DecimalInteger) { |i| @decimal_integer = i }
@@ -195,4 +196,8 @@ def test_decimal_numeric
     end
   end
 
+  def test_array
+    assert_equal(%w"", no_error {@opt.parse!(%w"--array a,b,c")})
+    assert_equal(%w"a b c", @array)
+  end
 end
diff --git a/test/optparse/test_optarg.rb b/test/optparse/test_optarg.rb
index 81127a8..f944605 100644
--- a/test/optparse/test_optarg.rb
+++ b/test/optparse/test_optarg.rb
@@ -9,6 +9,8 @@ def setup
     @opt.def_option("--regexp[=REGEXP]", Regexp) {|x| @reopt = x}
     @opt.def_option "--with_underscore[=VAL]" do |x| @flag = x end
     @opt.def_option "--with-hyphen[=VAL]" do |x| @flag = x end
+    @opt.def_option("--fallback[=VAL]") do |x = "fallback"| @flag = x end
+    @opt.def_option("--lambda[=VAL]", &->(x) {@flag = x})
     @reopt = nil
   end
 
@@ -57,4 +59,18 @@ def test_hyphenize
     assert_equal(%w"", no_error {@opt.parse!(%w"--with_hyphen=foo4")})
     assert_equal("foo4", @flag)
   end
+
+  def test_default_argument
+    assert_equal(%w"", no_error {@opt.parse!(%w"--fallback=val1")})
+    assert_equal("val1", @flag)
+    assert_equal(%w"", no_error {@opt.parse!(%w"--fallback")})
+    assert_equal("fallback", @flag)
+  end
+
+  def test_lambda
+    assert_equal(%w"", no_error {@opt.parse!(%w"--lambda=lambda1")})
+    assert_equal("lambda1", @flag)
+    assert_equal(%w"", no_error {@opt.parse!(%w"--lambda")})
+    assert_equal(nil, @flag)
+  end
 end
diff --git a/test/optparse/test_optparse.rb b/test/optparse/test_optparse.rb
index be9bcb8..3b9ccc7 100644
--- a/test/optparse/test_optparse.rb
+++ b/test/optparse/test_optparse.rb
@@ -73,10 +73,17 @@ def test_into
     @opt.def_option "-p", "--port=PORT", "port", Integer
     @opt.def_option "-v", "--verbose" do @verbose = true end
     @opt.def_option "-q", "--quiet" do @quiet = true end
+    @opt.def_option "-o", "--option [OPT]" do |opt| @option = opt end
     result = {}
     @opt.parse %w(--host localhost --port 8000 -v), into: result
     assert_equal({host: "localhost", port: 8000, verbose: true}, result)
     assert_equal(true, @verbose)
+    result = {}
+    @opt.parse %w(--option -q), into: result
+    assert_equal({quiet: true, option: nil}, result)
+    result = {}
+    @opt.parse %w(--option OPTION -v), into: result
+    assert_equal({verbose: true, option: "OPTION"}, result)
   end
 
   def test_require_exact
diff --git a/test/optparse/test_placearg.rb b/test/optparse/test_placearg.rb
index ed0e4d3..a8a11e6 100644
--- a/test/optparse/test_placearg.rb
+++ b/test/optparse/test_placearg.rb
@@ -13,6 +13,8 @@ def setup
     @reopt = nil
     @opt.def_option "--with_underscore=VAL" do |x| @flag = x end
     @opt.def_option "--with-hyphen=VAL" do |x| @flag = x end
+    @opt.def_option("--fallback [VAL]") do |x = "fallback"| @flag = x end
+    @opt.def_option("--lambda [VAL]", &->(x) {@flag = x})
   end
 
   def test_short
@@ -73,4 +75,22 @@ def test_conv
     assert_equal(%w"te.rb", no_error('[ruby-dev:38333]') {@opt.parse!(%w"-T1 te.rb")})
     assert_equal(1, @topt)
   end
+
+  def test_default_argument
+    assert_equal(%w"", no_error {@opt.parse!(%w"--fallback=val1")})
+    assert_equal("val1", @flag)
+    assert_equal(%w"", no_error {@opt.parse!(%w"--fallback val2")})
+    assert_equal("val2", @flag)
+    assert_equal(%w"", no_error {@opt.parse!(%w"--fallback")})
+    assert_equal("fallback", @flag)
+  end
+
+  def test_lambda
+    assert_equal(%w"", no_error {@opt.parse!(%w"--lambda=lambda1")})
+    assert_equal("lambda1", @flag)
+    assert_equal(%w"", no_error {@opt.parse!(%w"--lambda lambda2")})
+    assert_equal("lambda2", @flag)
+    assert_equal(%w"", no_error {@opt.parse!(%w"--lambda")})
+    assert_equal(nil, @flag)
+  end
 end
diff --git a/test/optparse/test_reqarg.rb b/test/optparse/test_reqarg.rb
index d5686d1..31d4fef 100644
--- a/test/optparse/test_reqarg.rb
+++ b/test/optparse/test_reqarg.rb
@@ -6,6 +6,7 @@ def setup
     super
     @opt.def_option "--with_underscore=VAL" do |x| @flag = x end
     @opt.def_option "--with-hyphen=VAL" do |x| @flag = x end
+    @opt.def_option("--lambda=VAL", &->(x) {@flag = x})
   end
 
   class Def1 < TestOptionParser
@@ -81,6 +82,11 @@ def test_hyphenize
     assert_equal("foo4", @flag)
   end
 
+  def test_lambda
+    assert_equal(%w"", no_error {@opt.parse!(%w"--lambda=lambda1")})
+    assert_equal("lambda1", @flag)
+  end
+
   class TestOptionParser::WithPattern < TestOptionParser
     def test_pattern
       pat = num = nil