Added null

pkg/yqlib/treeops/lib.go

@@ -72,6 +72,8 @@ func CreateValueOperation(value interface{}, stringValue string) *Operation {
 		node.Tag = "!!bool"
 	case string:
 		node.Tag = "!!str"
+	case nil:
+		node.Tag = "!!null"
 	}
 
 	return &Operation{

pkg/yqlib/treeops/operator_select_test.go

@@ -35,6 +35,12 @@ var selectOperatorScenarios = []expressionScenario{
 			"D0, P[a things], (!!map)::{include: true}\n",
 			"D0, P[a andMe], (!!map)::{include: fold}\n",
 		},
+	}, {
+		document:   `[cat,~,dog]`,
+		expression: `.[] | select(. == ~)`,
+		expected: []string{
+			"D0, P[1], (!!null)::~\n",
+		},
 	},
 }
 

pkg/yqlib/treeops/operator_value_test.go

@@ -67,6 +67,20 @@ var valueOperatorScenarios = []expressionScenario{
 			"D0, P[], (!!bool)::false\n",
 		},
 	},
+	{
+		document:   ``,
+		expression: `Null`,
+		expected: []string{
+			"D0, P[], (!!null)::Null\n",
+		},
+	},
+	{
+		document:   ``,
+		expression: `~`,
+		expected: []string{
+			"D0, P[], (!!null)::~\n",
+		},
+	},
 }
 
 func TestValueOperatorScenarios(t *testing.T) {

pkg/yqlib/treeops/path_tokeniser.go

@@ -151,6 +151,12 @@ func stringValue(wrapped bool) lex.Action {
 	}
 }
 
+func nullValue() lex.Action {
+	return func(s *lex.Scanner, m *machines.Match) (interface{}, error) {
+		return &Token{TokenType: OperationToken, Operation: CreateValueOperation(nil, string(m.Bytes))}, nil
+	}
+}
+
 func selfToken() lex.Action {
 	return func(s *lex.Scanner, m *machines.Match) (interface{}, error) {
 		op := &Operation{OperationType: SelfReference}
@@ -200,6 +206,9 @@ func initLexer() (*lex.Lexer, error) {
 	lexer.Add([]byte(`[Tt][Rr][Uu][Ee]`), booleanValue(true))
 	lexer.Add([]byte(`[Ff][Aa][Ll][Ss][Ee]`), booleanValue(false))
 
+	lexer.Add([]byte(`[Nn][Uu][Ll][Ll]`), nullValue())
+	lexer.Add([]byte(`~`), nullValue())
+
 	lexer.Add([]byte(`"[^ "]+"`), stringValue(true))
 
 	lexer.Add([]byte(`\[`), literalToken(OpenCollect, false))