Add existential modifier to variables. Note that this is not preserved from queries, which record the binding using the variable name, without other qualities.

Author: gkellogg

lib/rdf/query/variable.rb

@@ -67,10 +67,11 @@ class Variable
     #   an optional variable value
     # @param [Boolean] distinguished (true)
     #   defaults to false, unless name looks like a bnode
-    def initialize(name = nil, value = nil, distinguished: true)
+    def initialize(name = nil, value = nil, distinguished: true, existential: false)
       @name  = (name || "g#{__id__.to_i.abs}").to_sym
       @value = value
       @distinguished = distinguished
+      @existential = existential
     end
 
     ##
@@ -124,6 +125,23 @@ def distinguished=(value)
       @distinguished = value
     end
 
+    ##
+    # Returns `true` if this variable is existential.
+    #
+    # @return [Boolean]
+    def existential?
+      @existential
+    end
+
+    ##
+    # Sets if variable is existential or univeresal.
+    # By default, variables are universal
+    #
+    # @return [Boolean]
+    def existential=(value)
+      @existential = value
+    end
+
     ##
     # Rebinds this variable to the given `value`.
     #
@@ -211,6 +229,7 @@ def ===(other)
     #
     # Non-distinguished variables are indicated with a double `??`
     #
+    # Existential variables are indicated using a single `$`, or with `$$` if also non-distinguished
     # @example
     #   v = Variable.new("a")
     #   v.to_s => '?a'
@@ -219,7 +238,7 @@ def ===(other)
     #
     # @return [String]
     def to_s
-      prefix = distinguished? ? '?' : "??"
+      prefix = distinguished? ? (existential? ? '$' : '?') : (existential? ? '$$' : '??')
       unbound? ? "#{prefix}#{name}" : "#{prefix}#{name}=#{value}"
     end
   end # Variable

spec/query_variable_spec.rb

@@ -29,6 +29,22 @@
       end
     end
 
+    context "existential" do
+      subject { var.existential = true; var }
+      it "is existential" do
+        expect(subject).to be_existential
+      end
+
+      it "can be made universal" do
+        subject.existential = false
+        expect(subject).not_to be_existential
+      end
+
+      it "has a string representation" do
+        expect(subject.to_s).to eq "$x"
+      end
+    end
+    
     context "non-distinguished" do
       subject { var.distinguished = false; var }
       it "is nondistinguished" do
@@ -47,6 +63,22 @@
       it "has a string representation" do
         expect(subject.to_s).to eq "??x"
       end
+    
+      context "existential" do
+        subject { var.existential = true; var.distinguished = false; var }
+        it "is existential" do
+          expect(subject).to be_existential
+        end
+
+        it "can be made universal" do
+          subject.existential = false
+          expect(subject).not_to be_existential
+        end
+
+        it "has a string representation" do
+          expect(subject.to_s).to eq "$$x"
+        end
+      end
     end
 
     it "is convertible to a symbol" do