🔧 Add Config#with(**attrs) to make child configs

lib/net/imap/config.rb

@@ -165,6 +165,24 @@ def update(**attrs)
         self
       end
 
+      # :call-seq:
+      #   with(**attrs) -> config
+      #   with(**attrs) {|config| } -> result
+      #
+      # Without a block, returns a new config which inherits from self.  With a
+      # block, yields the new config and returns the block's result.
+      #
+      # If no keyword arguments are given, an ArgumentError will be raised.
+      #
+      # If +self+ is frozen, the copy will also be frozen.
+      def with(**attrs)
+        attrs.empty? and
+          raise ArgumentError, "expected keyword arguments, none given"
+        copy = new(**attrs)
+        copy.freeze if frozen?
+        block_given? ? yield(copy) : copy
+      end
+
       # :call-seq: to_h -> hash
       #
       # Returns all config attributes in a hash.

test/net/imap/test_config.rb

@@ -212,4 +212,29 @@ class ConfigTest < Test::Unit::TestCase
     assert_same false, config.sasl_ir?     # unchanged
   end
 
+  test "#with" do
+    orig = Config.new(open_timeout: 123, sasl_ir: false)
+    assert_raise(ArgumentError) do
+      orig.with
+    end
+    copy = orig.with(open_timeout: 456, idle_response_timeout: 789)
+    refute copy.frozen?
+    assert_same orig, copy.parent
+    assert_equal 123, orig.open_timeout # unchanged
+    assert_equal 456, copy.open_timeout
+    assert_equal 789, copy.idle_response_timeout
+    vals = nil
+    result = orig.with(open_timeout: 99, idle_response_timeout: 88) do |c|
+      vals = [c.open_timeout, c.idle_response_timeout, c.frozen?]
+      :result
+    end
+    assert_equal :result, result
+    assert_equal [99, 88, false], vals
+    orig.freeze
+    result = orig.with(open_timeout: 11) do |c|
+      vals = [c.open_timeout, c.idle_response_timeout, c.frozen?]
+    end
+    assert_equal [11, 5, true], vals
+  end
+
 end