🔧🏷 Add type coercion for config attributes

This is implemented as another module included under the other two,
still based on overriding `attr_accessor`.  Types are only enforced by
the attr_writer methods.  Fortunately, rdoc isn't confused by keyword
arguments to `attr_accessor`, so the type can be added to that.

Currently, only `:boolean` and `Integer` are supported, but it should be
easy to add more.

lib/net/imap/config.rb

@@ -3,6 +3,7 @@
 
 require_relative "config/attr_accessors"
 require_relative "config/attr_inheritance"
+require_relative "config/attr_type_coercion"
 
 module Net
   class IMAP
@@ -48,14 +49,19 @@ def self.[](config) # :nodoc: unfinished API
 
       include AttrAccessors
       include AttrInheritance
+      include AttrTypeCoercion
 
       # The debug mode (boolean)
       #
       # | Starting with version | The default value is |
       # |-----------------------|----------------------|
       # | _original_            | +false+              |
-      attr_accessor :debug
-      alias debug? debug
+      attr_accessor :debug, type: :boolean
+
+      # method: debug?
+      # :call-seq: debug? -> boolean
+      #
+      # Alias for #debug
 
       # Seconds to wait until a connection is opened.
       #
@@ -65,15 +71,15 @@ def self.[](config) # :nodoc: unfinished API
       # | Starting with version | The default value is |
       # |-----------------------|----------------------|
       # | _original_            | +30+ seconds         |
-      attr_accessor :open_timeout
+      attr_accessor :open_timeout, type: Integer
 
       # Seconds to wait until an IDLE response is received, after
       # the client asks to leave the IDLE state.  See Net::IMAP#idle_done.
       #
       # | Starting with version | The default value is |
       # |-----------------------|----------------------|
       # | _original_            | +5+ seconds          |
-      attr_accessor :idle_response_timeout
+      attr_accessor :idle_response_timeout, type: Integer
 
       # Creates a new config object and initialize its attribute with +attrs+.
       #

lib/net/imap/config/attr_type_coercion.rb

@@ -0,0 +1,45 @@
+# frozen_string_literal: true
+
+module Net
+  class IMAP
+    class Config
+      # Adds a +type+ keyword parameter to +attr_accessor+, which enforces
+      # config attributes have valid types, for example: boolean, numeric,
+      # enumeration, non-nullable, etc.
+      module AttrTypeCoercion
+        # :stopdoc: internal APIs only
+
+        module Macros # :nodoc: internal API
+          def attr_accessor(attr, type: nil)
+            super(attr)
+            AttrTypeCoercion.attr_accessor(attr, type: type)
+          end
+        end
+        private_constant :Macros
+
+        def self.included(mod)
+          mod.extend Macros
+        end
+        private_class_method :included
+
+        def self.attr_accessor(attr, type: nil)
+          return unless type
+          if    :boolean == type then boolean attr
+          elsif Integer  == type then integer attr
+          else raise ArgumentError, "unknown type coercion %p" % [type]
+          end
+        end
+
+        def self.boolean(attr)
+          define_method :"#{attr}=" do |val| super !!val end
+          define_method :"#{attr}?" do send attr end
+        end
+
+        def self.integer(attr)
+          define_method :"#{attr}=" do |val| super Integer val end
+        end
+
+      end
+    end
+  end
+end

test/net/imap/test_config.rb

@@ -28,6 +28,28 @@ class ConfigTest < Test::Unit::TestCase
     refute config.debug?
   end
 
+  test "boolean type constraints and conversion" do
+    config = Config.new
+    config.debug = 111
+    assert_equal true, config.debug
+    config.debug = nil
+    assert_equal false, config.debug
+  end
+
+  test "integer type constraints and conversion" do
+    config = Config.new
+    config.open_timeout = "111"
+    assert_equal 111, config.open_timeout
+    config.open_timeout = 222.0
+    assert_equal 222, config.open_timeout
+    config.open_timeout = 333.3
+    assert_equal 333, config.open_timeout
+    assert_raise(ArgumentError) do
+      config.open_timeout = "444 NaN"
+    end
+    assert_equal 333, config.open_timeout
+  end
+
   test ".default" do
     default = Config.default
     assert default.equal?(Config.default)