jneen · November 30, 2014 06:51 · Nov 30, 2014
diff --git a/variant.rb b/variant.rb
@@ -0,0 +1,100 @@
+class Variant
+  class Caser
+    attr_reader :value
+    def initialize(variant)
+      @variant = variant
+      @invoked = false
+    end
+
+    def try_case(tag, &b)
+      if @variant.tag == tag
+        @invoked = true
+        @value = yield(*@variant.values)
+      end
+    end
+
+    def invoked?
+      @invoked
+    end
+  end
+
+  def self.spec
+    @spec ||= {}
+  end
+
+  def self.caser
+    @caser ||= Class.new(Caser)
+  end
+
+  attr_reader :values
+  def initialize(*values)
+    @values = values
+  end
+
+  def self.variant(tag, *names)
+    parent_class = self
+    klass = spec[tag] = Class.new(self)
+    klass.class_eval do
+      names.each_with_index do |name, i|
+        define_method(name) { @values[i] }
+      end
+
+      define_method(:type) { parent_class }
+      define_method(:tag) { tag }
+    end
+
+    caser.class_eval do
+      define_method(tag) { |&b| try_case(tag, &b) }
+    end
+
+    (class << self; self; end).class_eval do
+      define_method(tag) { |*values| klass.new(*values) }
+    end
+  end
+
+  def inspect
+    "#<#{type}.#{tag}(#{values.map(&:inspect).join(', ')})>"
+  end
+
+  def cases(cases={}, &b)
+    if block_given?
+      caser = self.class.caser.new(self)
+      yield caser
+      if caser.invoked?
+        caser.value
+      else
+        non_exhaustive_cases!
+      end
+    else
+      selection = cases[tag] || cases[:else] || non_exhaustive_cases!
+      selection.call(*values)
+    end
+  end
+end
+
+class Order < Variant
+  variant :delivery, :address
+  variant :digital, :email
+  variant :pickup, :store_id
+end
+
+# n constructors
+# Order.delivery('123 Main St')
+# Order.digital('customer@example.com')
+# Order.pickup(456)
+# 
+# order.cases(
+#   delivery: ->(address) { "delivering to #{address}" },
+#   digital: ->(email) { "emailing to #{email}" },
+#   pickup: ->(store_id) {
+#     "picking up from #{Store.find(store_id).address}"
+#   },
+# )
+# 
+# order.cases do |c|
+#   c.delivery { |address| "delivering to #{address}" }
+#   c.digital { |email| "emailing to #{email}" }
+#   c.pickup do |store_id|
+#     "picking up from #{Store.find(store_id).address}"
+#   end
+# end