Support parameters with lazily-evaluated default values

Lazily-evaluated defaults can be provided by supplying `Value.lazy { ... }` as a
default. The lazy block will be evaluated at each instantiation time.

lib/safe_values/version.rb

@@ -1,3 +1,3 @@
 module SafeValues
-  VERSION = "1.0.2"
+  VERSION = "1.1.0"
 end

lib/value.rb

@@ -13,7 +13,9 @@
 # `ValueType = Value.new(:a, :b, c: default_value)`. The default values to
 # optional arguments are saved at class creation time and supplied as default
 # constructor arguments to instances. Default values are aliased, so providing
-# mutable defaults is discouraged.
+# mutable defaults is discouraged. Lazily-evaluated defaults can be provided by
+# supplying `Value.lazy { ... }` as a default. The lazy block will be evaluated
+# at each instantiation time.
 #
 # Two instance constructors are provided, with positional and keyword arguments.
 #
@@ -25,6 +27,8 @@
 # Value types may be constructed with keyword arguments using `with`.
 # For example: `ValueType.with(a: 1, b: 2, c: 3)`
 class Value < Struct
+  LazyDefault = Struct.new(:proc)
+
   class << self
     def new(*required_args, **optional_args, &block)
       arguments = {}
@@ -34,13 +38,24 @@ def new(*required_args, **optional_args, &block)
 
       clazz = super(*arguments.keys, &nil)
 
+      # Undefine setters
+      arguments.keys.each do |arg|
+        clazz.send(:undef_method, :"#{arg}=")
+      end
+
       # define class and instance methods in modules so that the class can
       # override them
       keyword_constructor = generate_keyword_constructor(arguments)
       class_method_module = Module.new do
         module_eval(keyword_constructor)
-        define_method(:__constructor_default) do |name|
-          optional_args.fetch(name)
+
+        optional_args.each do |name, value|
+          method_name = :"__constructor_default_#{name}"
+          if value.is_a?(LazyDefault)
+            define_method(method_name, &value.proc)
+          else
+            define_method(method_name) { value }
+          end
         end
       end
       clazz.extend(class_method_module)
@@ -57,6 +72,21 @@ def new(*required_args, **optional_args, &block)
       clazz
     end
 
+    def lazy(&proc)
+      LazyDefault.new(proc)
+    end
+
+    # Value classes by default create immutable instances. In some cases we may
+    # want the construction advantages of Values without the immutability
+    # constraint. Allow subclasses to override this behaviour.
+    def mutable?
+      defined?(@mutable) && @mutable
+    end
+
+    def mutable!
+      @mutable = true
+    end
+
     private
 
     def validate_names(*params)
@@ -73,23 +103,23 @@ def validate_names(*params)
     #
     # For a Value.new(:a, b: x), will define the method:
     #
-    # def initialize(a, b = self.class.__constructor_default(:b))
+    # def initialize(a, b = self.class.__constructor_default_b)
     #   super(a, b)
-    #   freeze
+    #   freeze unless self.class.mutable?
     # end
     def generate_constructor(arguments)
       params = arguments.map do |arg_name, required|
         if required
           arg_name
         else
-          "#{arg_name} = self.class.__constructor_default(:#{arg_name})"
+          "#{arg_name} = self.class.__constructor_default_#{arg_name}"
         end
       end
 
       <<-SRC
         def initialize(#{params.join(", ")})
           super(#{arguments.keys.join(", ")})
-          freeze
+          freeze unless self.class.mutable?
         end
       SRC
     end
@@ -100,15 +130,15 @@ def initialize(#{params.join(", ")})
     #
     # For a Value.new(:a, b: x), will define the (class) method:
     #
-    # def with(a:, b: __constructor_default(:b))
+    # def with(a:, b: __constructor_default_b)
     #   self.new(a, b)
     # end
     def generate_keyword_constructor(arguments)
       params = arguments.map do |arg_name, required|
         if required
           "#{arg_name}:"
         else
-          "#{arg_name}: __constructor_default(:#{arg_name})"
+          "#{arg_name}: __constructor_default_#{arg_name}"
         end
       end
 

spec/unit/value_spec.rb

@@ -90,6 +90,33 @@
     end
   end
 
+  context "with lazy defaults" do
+    let(:next_value) do
+      x = 0
+      ->() { x += 1 }
+    end
+
+    let(:value) { Value.new(a: Value.lazy(&next_value)) }
+
+    it "can construct an instance with positional lazy arguments" do
+      v = value.new
+      expect(v.a).to eq(1)
+      v = value.new
+      expect(v.a).to eq(2)
+      v = value.new(-1)
+      expect(v.a).to eq(-1)
+    end
+
+    it "can construct an instance with keyword lazy arguments" do
+      v = value.with()
+      expect(v.a).to eq(1)
+      v = value.with()
+      expect(v.a).to eq(2)
+      v = value.with(a: -1)
+      expect(v.a).to eq(-1)
+    end
+  end
+
   context "with a class body" do
     let(:value) do
       Value.new(:a) do