Add nil coalescing operator

checker/checker.go

@@ -38,9 +38,6 @@ func Check(tree *parser.Tree, config *conf.Config) (t reflect.Type, err error) {
 			}
 		default:
 			if t != nil {
-				if t.Kind() == reflect.Interface {
-					t = t.Elem()
-				}
 				if t.Kind() == v.config.Expect {
 					return t, nil
 				}
@@ -358,6 +355,21 @@ func (v *visitor) BinaryNode(node *ast.BinaryNode) (reflect.Type, info) {
 			return ret, info{}
 		}
 
+	case "??":
+		if l == nil && r != nil {
+			return r, info{}
+		}
+		if l != nil && r == nil {
+			return l, info{}
+		}
+		if l == nil && r == nil {
+			return nilType, info{}
+		}
+		if r.AssignableTo(l) {
+			return l, info{}
+		}
+		return anyType, info{}
+
 	default:
 		return v.error(node, "unknown operator (%v)", node.Operator)
 

checker/checker_test.go

@@ -121,6 +121,8 @@ var successTests = []string{
 	"Duration + Any == Time",
 	"Any + Duration == Time",
 	"Any.A?.B == nil",
+	"(Any.Bool ?? Bool) > 0",
+	"Bool ?? Bool",
 }
 
 func TestCheck(t *testing.T) {

compiler/compiler.go

@@ -424,6 +424,13 @@ func (c *compiler) BinaryNode(node *ast.BinaryNode) {
 		c.compile(node.Right)
 		c.emit(OpRange)
 
+	case "??":
+		c.compile(node.Left)
+		end := c.emit(OpJumpIfNotNil, placeholder)
+		c.emit(OpPop)
+		c.compile(node.Right)
+		c.patchJump(end)
+
 	default:
 		panic(fmt.Sprintf("unknown operator (%v)", node.Operator))
 

compiler/compiler_test.go

@@ -230,6 +230,25 @@ func TestCompile(t *testing.T) {
 				Arguments: []int{0, 1, 0, 2},
 			},
 		},
+		{
+			`A ?? 1`,
+			vm.Program{
+				Constants: []interface{}{
+					&runtime.Field{
+						Index: []int{0},
+						Path:  []string{"A"},
+					},
+					1,
+				},
+				Bytecode: []vm.Opcode{
+					vm.OpLoadField,
+					vm.OpJumpIfNotNil,
+					vm.OpPop,
+					vm.OpPush,
+				},
+				Arguments: []int{0, 2, 0, 1},
+			},
+		},
 	}
 
 	for _, test := range tests {

docs/Language-Definition.md

@@ -79,7 +79,7 @@ d>
     <tr>
         <td>Conditional</td>
         <td>
-            <code>?:</code> (ternary)
+            <code>?:</code> (ternary), <code>??</code> (nil coalescing)
         </td>
     </tr>
     <tr>
@@ -147,6 +147,15 @@ without checking if the struct or the map is `nil`. If the struct or the map is
 author?.User?.Name
 ```
 
+#### Nil coalescing
+
+The `??` operator can be used to return the left-hand side if it is not `nil`,
+otherwise the right-hand side is returned.
+
+```c++
+author?.User?.Name ?? "Anonymous"
+```
+
 ### Slice Operator
 
 The slice operator `[:]` can be used to access a slice of an array.

expr_test.go

@@ -1710,6 +1710,42 @@ func TestFunction(t *testing.T) {
 	assert.Equal(t, 20, out)
 }
 
+// Nil coalescing operator
+func TestRun_NilCoalescingOperator(t *testing.T) {
+	env := map[string]interface{}{
+		"foo": map[string]interface{}{
+			"bar": "value",
+		},
+	}
+
+	t.Run("value", func(t *testing.T) {
+		p, err := expr.Compile(`foo.bar ?? "default"`, expr.Env(env))
+		assert.NoError(t, err)
+
+		out, err := expr.Run(p, env)
+		assert.NoError(t, err)
+		assert.Equal(t, "value", out)
+	})
+
+	t.Run("default", func(t *testing.T) {
+		p, err := expr.Compile(`foo.baz ?? "default"`, expr.Env(env))
+		assert.NoError(t, err)
+
+		out, err := expr.Run(p, env)
+		assert.NoError(t, err)
+		assert.Equal(t, "default", out)
+	})
+
+	t.Run("default with chain", func(t *testing.T) {
+		p, err := expr.Compile(`foo?.bar ?? "default"`, expr.Env(env))
+		assert.NoError(t, err)
+
+		out, err := expr.Run(p, map[string]interface{}{})
+		assert.NoError(t, err)
+		assert.Equal(t, "default", out)
+	})
+}
+
 // Mock types
 
 type mockEnv struct {

parser/lexer/lexer_test.go

@@ -180,6 +180,15 @@ var lexTests = []lexTest{
 			{Kind: EOF},
 		},
 	},
+	{
+		`foo ?? bar`,
+		[]Token{
+			{Kind: Identifier, Value: "foo"},
+			{Kind: Operator, Value: "??"},
+			{Kind: Identifier, Value: "bar"},
+			{Kind: EOF},
+		},
+	},
 }
 
 func compareTokens(i1, i2 []Token) bool {

parser/lexer/state.go

@@ -156,7 +156,7 @@ func not(l *lexer) stateFn {
 }
 
 func questionMark(l *lexer) stateFn {
-	l.accept(".")
+	l.accept(".?")
 	l.emit(Operator)
 	return root
 }

parser/parser.go

@@ -58,6 +58,7 @@ var binaryOperators = map[string]operator{
 	"%":          {60, left},
 	"**":         {100, right},
 	"^":          {100, right},
+	"??":         {500, left},
 }
 
 var builtins = map[string]builtin{
@@ -113,9 +114,13 @@ func Parse(input string) (*Tree, error) {
 }
 
 func (p *parser) error(format string, args ...interface{}) {
+	p.errorAt(p.current, format, args...)
+}
+
+func (p *parser) errorAt(token Token, format string, args ...interface{}) {
 	if p.err == nil { // show first error
 		p.err = &file.Error{
-			Location: p.current.Location,
+			Location: token.Location,
 			Message:  fmt.Sprintf(format, args...),
 		}
 	}
@@ -143,22 +148,28 @@ func (p *parser) expect(kind Kind, values ...string) {
 func (p *parser) parseExpression(precedence int) Node {
 	nodeLeft := p.parsePrimary()
 
-	token := p.current
-	for token.Is(Operator) && p.err == nil {
+	lastOperator := ""
+	opToken := p.current
+	for opToken.Is(Operator) && p.err == nil {
 		negate := false
 		var notToken Token
 
-		if token.Is(Operator, "not") {
+		if opToken.Is(Operator, "not") {
 			p.next()
 			notToken = p.current
 			negate = true
-			token = p.current
+			opToken = p.current
 		}
 
-		if op, ok := binaryOperators[token.Value]; ok {
+		if op, ok := binaryOperators[opToken.Value]; ok {
 			if op.precedence >= precedence {
 				p.next()
 
+				if lastOperator == "??" && opToken.Value != "??" && !opToken.Is(Bracket, "(") {
+					p.errorAt(opToken, "Operator (%v) and coalesce expressions (??) cannot be mixed. Wrap either by parentheses.", opToken.Value)
+					break
+				}
+
 				var nodeRight Node
 				if op.associativity == left {
 					nodeRight = p.parseExpression(op.precedence + 1)
@@ -167,11 +178,11 @@ func (p *parser) parseExpression(precedence int) Node {
 				}
 
 				nodeLeft = &BinaryNode{
-					Operator: token.Value,
+					Operator: opToken.Value,
 					Left:     nodeLeft,
 					Right:    nodeRight,
 				}
-				nodeLeft.SetLocation(token.Location)
+				nodeLeft.SetLocation(opToken.Location)
 
 				if negate {
 					nodeLeft = &UnaryNode{
@@ -181,7 +192,8 @@ func (p *parser) parseExpression(precedence int) Node {
 					nodeLeft.SetLocation(notToken.Location)
 				}
 
-				token = p.current
+				lastOperator = opToken.Value
+				opToken = p.current
 				continue
 			}
 		}

parser/parser_test.go

@@ -408,6 +408,42 @@ func TestParse(t *testing.T) {
 			"[]",
 			&ArrayNode{},
 		},
+		{
+			"foo ?? bar",
+			&BinaryNode{Operator: "??",
+				Left:  &IdentifierNode{Value: "foo"},
+				Right: &IdentifierNode{Value: "bar"}},
+		},
+		{
+			"foo ?? bar ?? baz",
+			&BinaryNode{Operator: "??",
+				Left: &BinaryNode{Operator: "??",
+					Left:  &IdentifierNode{Value: "foo"},
+					Right: &IdentifierNode{Value: "bar"}},
+				Right: &IdentifierNode{Value: "baz"}},
+		},
+		{
+			"foo ?? (bar || baz)",
+			&BinaryNode{Operator: "??",
+				Left: &IdentifierNode{Value: "foo"},
+				Right: &BinaryNode{Operator: "||",
+					Left:  &IdentifierNode{Value: "bar"},
+					Right: &IdentifierNode{Value: "baz"}}},
+		},
+		{
+			"foo || bar ?? baz",
+			&BinaryNode{Operator: "||",
+				Left: &IdentifierNode{Value: "foo"},
+				Right: &BinaryNode{Operator: "??",
+					Left:  &IdentifierNode{Value: "bar"},
+					Right: &IdentifierNode{Value: "baz"}}},
+		},
+		{
+			"foo ?? bar()",
+			&BinaryNode{Operator: "??",
+				Left:  &IdentifierNode{Value: "foo"},
+				Right: &CallNode{Callee: &IdentifierNode{Value: "bar"}}},
+		},
 	}
 	for _, test := range parseTests {
 		actual, err := parser.Parse(test.input)
@@ -479,6 +515,11 @@ a map key must be a quoted string, a number, a identifier, or an expression encl
 unexpected token Operator(",") (1:16)
  | {foo:1, bar:2, ,}
  | ...............^
+
+foo ?? bar || baz
+Operator (||) and coalesce expressions (??) cannot be mixed. Wrap either by parentheses. (1:12)
+ | foo ?? bar || baz
+ | ...........^
 `
 
 func TestParse_error(t *testing.T) {

vm/opcodes.go

@@ -26,6 +26,7 @@ const (
 	OpJumpIfTrue
 	OpJumpIfFalse
 	OpJumpIfNil
+	OpJumpIfNotNil
 	OpJumpIfEnd
 	OpJumpBackward
 	OpIn

vm/program.go

@@ -138,6 +138,9 @@ func (program *Program) Disassemble() string {
 		case OpJumpIfNil:
 			jump("OpJumpIfNil")
 
+		case OpJumpIfNotNil:
+			jump("OpJumpIfNotNil")
+
 		case OpJumpIfEnd:
 			jump("OpJumpIfEnd")
 

vm/vm.go

@@ -174,6 +174,11 @@ func (vm *VM) Run(program *Program, env interface{}) (_ interface{}, err error)
 				vm.ip += arg
 			}
 
+		case OpJumpIfNotNil:
+			if !runtime.IsNil(vm.current()) {
+				vm.ip += arg
+			}
+
 		case OpJumpIfEnd:
 			scope := vm.Scope()
 			if scope.It >= scope.Len {