start infix Operators

<expression> <infix operator> <expression>
eight infix operators:
 5 + 5
 5 - 5
 5 * 5
 5 / 5
 5 > 5
 5 < 5
 5 == 5
 5 != 5

ast/ast.go

@@ -181,3 +181,27 @@ func (pe *PrefixExpression) String() string {
 }
 
 func (pe *PrefixExpression) expressionNode() {}
+
+// <expression> <infix operator> <expression>
+type InfixExpression struct {
+	Token    token.Token // The operator token, e.g. + - * /
+	Left     Expression
+	Operator string
+	Right    Expression
+}
+
+func (oe *InfixExpression) TokenLiteral() string { return oe.Token.Literal }
+
+func (oe *InfixExpression) String() string {
+	var out bytes.Buffer
+
+	out.WriteString("(")
+	out.WriteString(oe.Left.String())
+	out.WriteString(" " + oe.Operator + " ")
+	out.WriteString(oe.Right.String())
+	out.WriteString(")")
+
+	return out.String()
+}
+
+func (oe *InfixExpression) expressionNode() {}

parser/parser.go

@@ -27,6 +27,18 @@ const (
 	CALL        // myFunction(X)
 )
 
+// 指派 token 类型的优先级
+var precedences = map[token.TypeToken]int{
+	token.EQ:       EQUALS,
+	token.NOT_EQ:   EQUALS,
+	token.GT:       LESSGREATER,
+	token.LT:       LESSGREATER,
+	token.PLUS:     SUM,
+	token.MINUS:    SUM,
+	token.ASTERISK: PRODUCT,
+	token.SLASH:    PRODUCT,
+}
+
 type (
 	prefixParseFn func() ast.Expression
 	infixParseFn  func(expression ast.Expression) ast.Expression
@@ -55,6 +67,16 @@ func New(l *lexer.Lexer) *Parser {
 	p.registerPrefix(token.BANG, p.parsePrefixExpression)
 	p.registerPrefix(token.MINUS, p.parsePrefixExpression)
 
+	p.infixParseFns = make(map[token.TypeToken]infixParseFn)
+	p.registerInfix(token.PLUS, p.parseInfixExpression)
+	p.registerInfix(token.MINUS, p.parseInfixExpression)
+	p.registerInfix(token.ASTERISK, p.parseInfixExpression)
+	p.registerInfix(token.SLASH, p.parseInfixExpression)
+	p.registerInfix(token.EQ, p.parseInfixExpression)
+	p.registerInfix(token.NOT_EQ, p.parseInfixExpression)
+	p.registerInfix(token.GT, p.parseInfixExpression)
+	p.registerInfix(token.LT, p.parseInfixExpression)
+
 	// Read two tokens, so curToken and peekToken are both set
 	p.nextToken()
 	p.nextToken()
@@ -78,6 +100,19 @@ func (p *Parser) parseIntegerLiteral() ast.Expression {
 
 	return lit
 }
+func (p *Parser) parseInfixExpression(left ast.Expression) ast.Expression {
+	exp := &ast.InfixExpression{
+		Token:    p.curToken,
+		Left:     left,
+		Operator: p.curToken.Literal,
+	}
+
+	precedence := p.curPrecedence()
+	p.nextToken()
+	exp.Right = p.parseExpression(precedence)
+
+	return exp
+}
 func (p *Parser) parsePrefixExpression() ast.Expression {
 	exp := &ast.PrefixExpression{
 		Token:    p.curToken,
@@ -190,9 +225,18 @@ func (p *Parser) parseExpression(precedence int) ast.Expression {
 		p.noPrefixParseFnError(p.curToken.Type)
 		return nil
 	}
-
 	leftExp := prefix()
 
+	for !p.peekTokenIs(token.SEMICOLON) && precedence < p.peekPrecedence() {
+		infix := p.infixParseFns[p.peekToken.Type]
+		if infix == nil {
+			return leftExp
+		}
+		p.nextToken()
+
+		leftExp = infix(leftExp)
+	}
+
 	return leftExp
 }
 
@@ -226,3 +270,15 @@ func (p *Parser) noPrefixParseFnError(t token.TypeToken) {
 	msg := fmt.Sprintf("no prefix parse function for %s found", t)
 	p.errors = append(p.errors, msg)
 }
+func (p *Parser) peekPrecedence() int {
+	if p, ok := precedences[p.peekToken.Type]; ok {
+		return p
+	}
+	return LOWEST
+}
+func (p *Parser) curPrecedence() int {
+	if p, ok := precedences[p.curToken.Type]; ok {
+		return p
+	}
+	return LOWEST
+}

parser/parser_test.go

@@ -238,3 +238,124 @@ func testIntegerLiteral(t *testing.T, il ast.Expression, value int64) bool {
 	}
 	return true
 }
+
+// <expression> <infix operator> <expression>
+func TestParsingInfixExpressions(t *testing.T) {
+	infixTests := []struct {
+		input      string
+		leftValue  int64
+		operator   string
+		rightValue int64
+	}{
+		{"5 + 5", 5, "+", 5},
+		{"5 - 5", 5, "-", 5},
+		{"5 * 5", 5, "*", 5},
+		{"5 / 5", 5, "/", 5},
+		{"5 > 5", 5, ">", 5},
+		{"5 < 5", 5, "<", 5},
+		{"5 == 5", 5, "==", 5},
+		{"5 != 5", 5, "!=", 5},
+	}
+
+	for _, tt := range infixTests {
+		l := lexer.New(tt.input)
+		p := New(l)
+		program := p.ParseProgram()
+		checkParseErrors(t, p)
+
+		if len(program.Statements) != 1 {
+			t.Fatalf("program.Statements does not contain %d statements. got=%d\n",
+				1, len(program.Statements))
+		}
+		stmt, ok := program.Statements[0].(*ast.ExpressionStatement)
+		if !ok {
+			t.Fatalf("program.Statements[0] is not ast.ExpressionStatement. got=%T",
+				program.Statements[0])
+		}
+		exp, ok := stmt.Expression.(*ast.InfixExpression)
+		if !ok {
+			t.Fatalf("exp is not ast.InfixExpression. got =%T", stmt.Expression)
+		}
+		if !testIntegerLiteral(t, exp.Left, tt.leftValue) {
+			return
+		}
+		if exp.Operator != tt.operator {
+			t.Fatalf("exp.Operator is not '%s'. got=%s", tt.operator, exp.Operator)
+		}
+		if !testIntegerLiteral(t, exp.Right, tt.rightValue) {
+			return
+		}
+	}
+}
+
+func TestOperatorPrecedenceParsing(t *testing.T) {
+	tests := []struct {
+		input    string
+		expected string
+	}{
+		{
+			"-a * b",
+			"((-a) * b)",
+		},
+		{
+			"!-a",
+			"(!(-a))",
+		},
+		{
+			"a + b + c",
+			"((a + b) + c)",
+		},
+		{
+			"a + b - c",
+			"((a + b) - c)",
+		},
+		{
+			"a * b * c",
+			"((a * b) * c)",
+		},
+		{
+			"a * b / c",
+			"((a * b) / c)",
+		},
+		{
+			"a + b / c",
+			"(a + (b / c))",
+		},
+		{
+			"a + b * c + d / e - f",
+			"(((a + (b * c)) + (d / e)) - f)",
+		},
+		{
+			"3 + 4; -5 *5",
+			"(3 + 4)((-5) * 5)",
+		},
+		{
+			"5 > 4 == 3 < 4",
+			"((5 > 4) == (3 < 4))",
+		},
+		{
+			"5 < 4 != 3 > 4",
+			"((5 < 4) != (3 > 4))",
+		},
+		{
+			"3 + 4 * 5 == 3 * 1 + 4 * 5",
+			"((3 + (4 * 5)) == ((3 * 1) + (4 * 5)))",
+		},
+		{
+			"3 + 4 * 5 == 3 * 1 + 4 * 5",
+			"((3 + (4 * 5)) == ((3 * 1) + (4 * 5)))",
+		},
+	}
+
+	for _, tt := range tests {
+		l := lexer.New(tt.input)
+		p := New(l)
+		program := p.ParseProgram()
+		checkParseErrors(t, p)
+
+		actual := program.String()
+		if actual != tt.expected {
+			t.Errorf("exptected=%q, got=%q", tt.expected, actual)
+		}
+	}
+}