Implemented null coalesce operator

OperatorToken.go

@@ -38,6 +38,7 @@ const (
 
 	TERNARY_TRUE
 	TERNARY_FALSE
+	COALESCE
 )
 
 type OperatorPrecedence int
@@ -106,6 +107,8 @@ func findOperatorPrecedenceForSymbol(symbol OperatorSymbol) OperatorPrecedence {
 		fallthrough
 	case INVERT:
 		return PREFIX_PRECEDENCE
+	case COALESCE:
+		fallthrough
 	case TERNARY_TRUE:
 		fallthrough
 	case TERNARY_FALSE:
@@ -171,6 +174,7 @@ var PREFIX_SYMBOLS = map[string]OperatorSymbol{
 var TERNARY_SYMBOLS = map[string]OperatorSymbol{
 	"?": TERNARY_TRUE,
 	":": TERNARY_FALSE,
+	"??": COALESCE,
 }
 
 // this is defined separately from ADDITIVE_SYMBOLS et al because it's needed for parsing, not stage planning.
@@ -296,6 +300,8 @@ func (this OperatorSymbol) String() string {
 		return "?"
 	case TERNARY_FALSE:
 		return ":"
+	case COALESCE:
+		return "??"
 	}
 	return ""
 }

evaluation_test.go

@@ -263,6 +263,11 @@ func TestNoParameterEvaluation(test *testing.T) {
 			Input:    "1 * 4 / 2 * 8",
 			Expected: 16.0,
 		},
+		EvaluationTest{
+			Name:     "Null coalesce precedence",
+			Input:    "true ?? true ? 100 + 200 : 400",
+			Expected: 300.0,
+		},
 		EvaluationTest{
 
 			Name:     "Identical date equivalence",
@@ -437,6 +442,12 @@ func TestNoParameterEvaluation(test *testing.T) {
 			Input:    "true + 'bar' == 'truebar'",
 			Expected: true,
 		},
+		EvaluationTest{
+
+			Name:     "Null coalesce left",
+			Input:    "1 ?? 2",
+			Expected: 1.0,
+		},
 	}
 
 	runEvaluationTests(evaluationTests, test)
@@ -845,6 +856,18 @@ func TestParameterizedEvaluation(test *testing.T) {
 			},
 			Expected: 0.0,
 		},
+		EvaluationTest{
+
+			Name:  "Null coalesce right",
+			Input: "foo ?? 1.0",
+			Parameters: []EvaluationParameter{
+				EvaluationParameter{
+					Name:  "foo",
+					Value: nil,
+				},
+			},
+			Expected: 1.0,
+		},
 	}
 
 	runEvaluationTests(evaluationTests, test)

parsing_test.go

@@ -998,6 +998,25 @@ func TestTernaryParsing(test *testing.T) {
 				},
 			},
 		},
+		TokenParsingTest{
+
+			Name:  "Null coalesce left",
+			Input: "1 ?? 2",
+			Expected: []ExpressionToken{
+				ExpressionToken{
+					Kind:  NUMERIC,
+					Value: 1.0,
+				},
+				ExpressionToken{
+					Kind:  TERNARY,
+					Value: "??",
+				},
+				ExpressionToken{
+					Kind:  NUMERIC,
+					Value: 2.0,
+				},
+			},
+		},
 	}
 
 	runTokenParsingTest(tokenParsingTests, test)

stagePlanner.go

@@ -32,6 +32,7 @@ var stageSymbolMap = map[OperatorSymbol]evaluationOperator{
 	BITWISE_NOT:    bitwiseNotStage,
 	TERNARY_TRUE:   ternaryIfStage,
 	TERNARY_FALSE:  ternaryElseStage,
+	COALESCE:		ternaryElseStage,
 }
 
 /*
@@ -359,6 +360,8 @@ func findTypeChecks(symbol OperatorSymbol) typeChecks {
 		return typeChecks{}
 	case TERNARY_FALSE:
 		fallthrough
+	case COALESCE:
+		fallthrough
 	default:
 		return typeChecks{}
 	}