add empty

app/models/logic.rb

@@ -8,10 +8,12 @@ module Logic
   end
 
   def self.class_from_name(name)
-    [Constant]
+    [Constant, Empty]
       .find { |c| c.name == name }
   end
 
   def constant(value) = Logic::Constant.new(value)
 
+  def empty = Logic::Empty.new
+
 end

app/models/logic/empty.rb

@@ -0,0 +1,21 @@
+class Logic::Empty < Logic::Term
+  def to_s = "empty member"
+
+  def type = :empty
+
+  def errors(_stable_ids = nil) = ['empty']
+
+  def to_h
+    {
+      "op" => self.class.name
+    }
+  end
+
+  def self.from_h(_h)
+    self.new
+  end
+
+  def ==(other)
+    self.class == other.class
+  end
+end

spec/models/logic/empty_spec.rb

@@ -0,0 +1,16 @@
+describe Logic::Constant do
+  include Logic
+
+  describe '#type' do
+    it { expect(empty.type).to eq(:empty) }
+  end
+
+  describe '#errors' do
+    it { expect(empty.errors).to eq(['empty']) }
+  end
+
+  describe '#==' do
+    it { expect(empty).to eq(empty) }
+    it { expect(empty).not_to eq(constant(true)) }
+  end
+end

spec/models/logic_spec.rb

@@ -4,5 +4,7 @@ describe Logic do
   it 'serializes deserializes' do
     expect(Logic.from_h(constant(1).to_h)).to eq(constant(1))
     expect(Logic.from_json(constant(1).to_json)).to eq(constant(1))
+
+    expect(Logic.from_h(empty.to_h)).to eq(empty)
   end
 end