Add the concept of NULL

Allow NULL to be referenced in formulas, and for passed-in variables to
have the value of 'null', which is evaluated as nil.

README.md

@@ -125,7 +125,7 @@ BUILT-IN OPERATORS AND FUNCTIONS
 
 Math: `+ - * / %`
 
-Logic: `< > <= >= <> != = AND OR`
+Logic: `< > <= >= <> != = AND OR NULL`
 
 Functions: `IF NOT MIN MAX ROUND ROUNDDOWN ROUNDUP`
 

lib/dentaku/ast/identifier.rb

@@ -12,6 +12,8 @@ def initialize(token)
       def value(context={})
         v = context[identifier]
         case v
+        when :null
+          nil
         when Node
           v.value(context)
         when NilClass

lib/dentaku/calculator.rb

@@ -70,6 +70,10 @@ def store(key_or_hash, value=nil)
         memory[key_or_hash.to_s.downcase] = value
       end
 
+      memory.each do |key, value|
+        memory[key] = :null if value == 'null'
+      end
+
       if block_given?
         begin
           result = yield

lib/dentaku/parser.rb

@@ -54,6 +54,9 @@ def parse
             operations.push op_class
           end
 
+        when :null
+          output.push AST::Nil.new
+
         when :function
           arities.push 0
           operations.push function(token)

lib/dentaku/token_scanner.rb

@@ -26,6 +26,7 @@ def scan(string, last_token=nil)
     class << self
       def available_scanners
         [
+          :null,
           :whitespace,
           :numeric,
           :double_quoted_string,
@@ -68,6 +69,10 @@ def whitespace
         new(:whitespace, '\s+')
       end
 
+      def null
+        new(:null, 'null\b')
+      end
+
       def numeric
         new(:numeric, '(\d+(\.\d+)?|\.\d+)\b', lambda { |raw| raw =~ /\./ ? BigDecimal.new(raw) : raw.to_i })
       end

spec/calculator_spec.rb

@@ -260,6 +260,16 @@
     end
   end
 
+  describe 'explicit NULL' do
+    it 'can be used in IF statements' do
+      expect(calculator.evaluate('IF(null, 1, 2)')).to eq(2)
+    end
+
+    it 'can be used in IF statements when passed in' do
+      expect(calculator.evaluate('IF(foo, 1, 2)', foo: 'null')).to eq(2)
+    end
+  end
+
   describe 'case statements' do
     it 'handles complex then statements' do
       formula = <<-FORMULA

spec/parser_spec.rb

@@ -133,4 +133,10 @@
       described_class.new([five, times, minus]).parse
     }.to raise_error(Dentaku::ParseError)
   end
+
+  it "evaluates explicit 'NULL' as a Nil" do
+    null  = Dentaku::Token.new(:null, nil)
+    node = described_class.new([null]).parse
+    expect(node.value).to eq(nil)
+  end
 end

spec/token_scanner_spec.rb

@@ -28,7 +28,7 @@
   end
 
   it 'returns a list of all configured scanners' do
-    expect(described_class.scanners.length).to eq 13
+    expect(described_class.scanners.length).to eq 14
   end
 
   it 'allows customizing available scanners' do