parser: now checks whether the assign operator '=' is preceded by a variable

parser.go

@@ -76,6 +76,7 @@ func (self *parser) parseList(scanner *scanner) (tree *term, err error) {
 func (self *parser) parse(scanner *scanner, termSymbols ...Symbol) (tree *ast, err error) {
 	tree = NewAst()
 	firstToken := true
+	lastSym := SymUnknown
 	for tk := scanner.Next(); err == nil && tk != nil && !tk.IsTerm(termSymbols); tk = scanner.Next() {
 		if tk.Sym == SymComment {
 			continue
@@ -112,9 +113,16 @@ func (self *parser) parse(scanner *scanner, termSymbols ...Symbol) (tree *ast, e
 			if listTerm, err = self.parseList(scanner); err == nil {
 				err = tree.addTerm(listTerm)
 			}
+		case SymEqual:
+			if lastSym == SymIdentifier {
+				err = tree.addToken(tk)
+			} else {
+				err = fmt.Errorf(`assign operator (%q) must be preceded by a variable`, tk.source)
+			}
 		default:
 			err = tree.addToken(tk)
 		}
+		lastSym = tk.Sym
 	}
 	return
 }

parser_test.go

@@ -128,6 +128,9 @@ func TestParser(t *testing.T) {
 		/* 101 */ {`false or (x==2)`, nil, errors.New(`undefined variable "x"`)},
 		/* 102 */ {`a=5; a`, int64(5), nil},
 		/* 103 */ {`a=5; b=2; add(a, b*3)`, int64(11), nil},
+		/* 104 */ {`2=5`, nil, errors.New(`assign operator ("=") must be preceded by a variable`)},
+		/* 105 */ {`2+a=5`, nil, errors.New(`left operand of "=" must be a variable`)},
+		/* 106 */ {`2+(a=5)`, int64(7), nil},
 	}
 	succeeded := 0
 	failed := 0