Add Null-Coalescing(??) operator to the language and fix some other m…

…inor bugs.

src/monkey/eval/eval.go

@@ -1623,6 +1623,14 @@ func evalInfixExpression(node *ast.InfixExpression, left, right Object, scope *S
 		return evalInfixExpressionUDO(node, left, right, scope)
 	}
 
+	//Null-Coalescing Operator(??)
+	if node.Token.Type == token.QUESTIONMM {
+		if left.Type() == NIL_OBJ {
+			return right
+		}
+		return left
+	}
+
 	if isMetaOperators(node.Token.Type) {
 		return evalMetaOperatorInfixExpression(node, left, right, scope)
 	}

src/monkey/eval/goobject.go

@@ -213,6 +213,7 @@ func (gfn *GoFuncObject) CallMethod(line string, scope *Scope, method string, ar
 	outVals := reflect.ValueOf(gfn.fn).Call(inArgs)
 	// Convert the result back to monkey Object
 	for _, val := range outVals {
+		// Here we only retuns GoObject, See comment of 'RegisterVars'.
 		results = append(results, NewGoObject(val.Interface()))
 //		switch val.Kind() {
 //		case reflect.Bool:

src/monkey/lexer/lexer.go

@@ -341,6 +341,13 @@ func (l *Lexer) NextToken() token.Token {
 			} else {
 				tok = newToken(token.BITXOR, l.ch)
 			}
+		case token.QUESTIONM:
+			if l.peek() == '?' {
+			tok = token.Token{Type: token.QUESTIONMM, Literal: string(l.ch) + string(l.peek())}
+				l.readNext()
+			} else {
+				tok = newToken(token.QUESTIONM, l.ch)
+			}
 		default:
 			tok = newToken(t, l.ch)
 		}

src/monkey/parser/parser.go

@@ -34,6 +34,7 @@ const (
 	FATARROW
 	CONDOR
 	CONDAND
+	NULLCOALESCING //??
 	EQUALS
 	LESSGREATER
 	BITOR
@@ -76,6 +77,7 @@ var precedences = map[token.TokenType]int{
 	token.SHIFT_R:    SHIFTS,
 	token.COLON:      SLICE,
 	token.QUESTIONM:  TERNARY,
+	token.QUESTIONMM: NULLCOALESCING,
 	token.DOTDOT:     DOTDOT,
 	token.PLUS:       SUM,
 	token.MINUS:      SUM,
@@ -255,6 +257,7 @@ func (p *Parser) registerAction() {
 	p.registerInfix(token.BITOR, p.parseInfixExpression)
 	p.registerInfix(token.BITXOR, p.parseInfixExpression)
 	p.registerInfix(token.UDO, p.parseInfixExpression)
+	p.registerInfix(token.QUESTIONMM, p.parseInfixExpression)
 
 	p.registerInfix(token.ASSIGN, p.parseAssignExpression)
 	p.registerInfix(token.PLUS_A, p.parseAssignExpression)
@@ -762,6 +765,11 @@ func (p *Parser) parseLetStatement() *ast.LetStatement {
 		if p.curTokenIs(token.ASSIGN) || p.curTokenIs(token.SEMICOLON) {
 			break
 		}
+		if !p.curTokenIs(token.COMMA) {
+			msg := fmt.Sprintf("Syntax Error:%v- expected token to be comma, got %s instead.", p.curToken.Pos, p.curToken.Type)
+			p.errors = append(p.errors, msg)
+			return stmt
+		}
 	}
 
 	if p.curTokenIs(token.SEMICOLON) { //let x;

src/monkey/token/token.go

@@ -136,6 +136,8 @@ const (
 	TILDECARET    // ~^
 
 	USING
+	QUESTIONMM  // ?? (Null Coalescing Operator)
+
 )
 
 var keywords = map[string]TokenType{
@@ -369,6 +371,8 @@ func (tt TokenType) String() string {
 		return "THROW"
 	case QUESTIONM:
 		return "?"
+	case QUESTIONMM:
+		return "??"
 	case DEFER:
 		return "DEFER"
 	case NIL: