Parse if expressions

kamilturek · Jun 15, 2024 · df2bb9c · df2bb9c
1 parent b4daf99
commit df2bb9c
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Showing 3 changed files with 194 additions and 4 deletions.
diff --git a/ast/ast.go b/ast/ast.go
@@ -190,6 +190,56 @@ func (ie *InfixExpression) String() string {
 	return fmt.Sprintf("(%s %s %s)", ie.Left.String(), ie.Operator, ie.Right.String())
 }
 
+// If Expression
+
+type IfExpression struct {
+	Token       token.Token // the `if` token
+	Condition   Expression
+	Consequence *BlockStatement
+	Alternative *BlockStatement
+}
+
+func (ie *IfExpression) expressionNode() {}
+
+func (ie *IfExpression) TokenLiteral() string { return ie.Token.Literal }
+
+func (ie *IfExpression) String() string {
+	var out bytes.Buffer
+
+	out.WriteString("if")
+	out.WriteString(ie.Condition.String())
+	out.WriteString(" ")
+	out.WriteString(ie.Consequence.String())
+
+	if ie.Alternative != nil {
+		out.WriteString("else")
+		out.WriteString(ie.Alternative.String())
+	}
+
+	return out.String()
+}
+
+// Block Statement
+
+type BlockStatement struct {
+	Token      token.Token // the `{`` token
+	Statements []Statement
+}
+
+func (bs *BlockStatement) statementNode() {}
+
+func (bs *BlockStatement) TokenLiteral() string { return bs.Token.Literal }
+
+func (bs *BlockStatement) String() string {
+	var out bytes.Buffer
+
+	for _, s := range bs.Statements {
+		out.WriteString(s.String())
+	}
+
+	return out.String()
+}
+
 // Boolean
 
 type BooleanLiteral struct {

diff --git a/parser/parser.go b/parser/parser.go
@@ -61,6 +61,7 @@ func NewParser(l *lexer.Lexer) *Parser {
 	p.registerPrefix(token.BANG, p.parsePrefixExpression)
 	p.registerPrefix(token.MINUS, p.parsePrefixExpression)
 	p.registerPrefix(token.LPAREN, p.parseGroupedExpression)
+	p.registerPrefix(token.IF, p.parseIfExpression)
 
 	p.registerInfix(token.PLUS, p.parseInfixExpression)
 	p.registerInfix(token.MINUS, p.parseInfixExpression)
@@ -115,7 +116,7 @@ func (p *Parser) peekTokenIs(t token.TokenType) bool {
 	return p.peekToken.Type == t
 }
 
-func (p *Parser) exceptPeek(t token.TokenType) bool {
+func (p *Parser) expectPeek(t token.TokenType) bool {
 	if p.peekTokenIs(t) {
 		p.nextToken()
 
@@ -167,13 +168,13 @@ func (p *Parser) parseStatement() ast.Statement {
 func (p *Parser) parseLetStatement() *ast.LetStatement {
 	stmt := &ast.LetStatement{Token: p.curToken}
 
-	if !p.exceptPeek(token.IDENT) {
+	if !p.expectPeek(token.IDENT) {
 		return nil
 	}
 
 	stmt.Name = &ast.Identifier{Token: p.curToken, Value: p.curToken.Literal}
 
-	if !p.exceptPeek(token.ASSIGN) {
+	if !p.expectPeek(token.ASSIGN) {
 		return nil
 	}
 
@@ -265,7 +266,7 @@ func (p *Parser) parseGroupedExpression() ast.Expression {
 
 	exp := p.parseExpression(LOWEST)
 
-	if !p.exceptPeek(token.RPAREN) {
+	if !p.expectPeek(token.RPAREN) {
 		return nil
 	}
 
@@ -294,3 +295,49 @@ func (p *Parser) parseBooleanLiteral() ast.Expression {
 		Value: p.curTokenIs(token.TRUE),
 	}
 }
+
+func (p *Parser) parseIfExpression() ast.Expression {
+	expression := &ast.IfExpression{Token: p.curToken}
+
+	if !p.expectPeek(token.LPAREN) {
+		return nil
+	}
+
+	p.nextToken()
+	expression.Condition = p.parseExpression(LOWEST)
+
+	if !p.expectPeek(token.RPAREN) {
+		return nil
+	}
+
+	if !p.expectPeek(token.LBRACE) {
+		return nil
+	}
+
+	expression.Consequence = p.parseBlockStatement()
+
+	if p.peekTokenIs(token.ELSE) {
+		p.nextToken()
+
+		if !p.expectPeek(token.LBRACE) {
+			return nil
+		}
+
+		expression.Alternative = p.parseBlockStatement()
+	}
+
+	return expression
+}
+
+func (p *Parser) parseBlockStatement() *ast.BlockStatement {
+	block := &ast.BlockStatement{}
+
+	p.nextToken()
+
+	for !p.curTokenIs(token.RBRACE) && !p.curTokenIs(token.EOF) {
+		block.Statements = append(block.Statements, p.parseStatement())
+		p.nextToken()
+	}
+
+	return block
+}
diff --git a/parser/parser_test.go b/parser/parser_test.go
@@ -499,3 +499,96 @@ func TestBoolean(t *testing.T) {
 		testLiteralExpression(t, stmt.Expression, tt.expected)
 	}
 }
+
+func TestIfExpression(t *testing.T) {
+	input := `if (x < y) { x }`
+
+	l := lexer.NewLexer(input)
+	p := parser.NewParser(l)
+	program := p.ParseProgram()
+	checkParserErrors(t, p)
+
+	if len(program.Statements) != 1 {
+		t.Fatalf("program.Statements does not contain 1 statement. got=%d", len(program.Statements))
+	}
+
+	stmt, ok := program.Statements[0].(*ast.ExpressionStatement)
+	if !ok {
+		t.Fatalf("program.Statements[0] not *ast.ExpressionStatement. got=%T", program.Statements[0])
+	}
+
+	exp, ok := stmt.Expression.(*ast.IfExpression)
+	if !ok {
+		t.Fatalf("stmt.Expression not *ast.IfExpression. got=%T", stmt.Expression)
+	}
+
+	if !testInfixExpression(t, exp.Condition, "x", "<", "y") {
+		return
+	}
+
+	if len(exp.Consequence.Statements) != 1 {
+		t.Errorf("exp.Consequence.Statements is not 1 statements. got=%d", len(exp.Consequence.Statements))
+	}
+
+	consequence, ok := exp.Consequence.Statements[0].(*ast.ExpressionStatement)
+	if !ok {
+		t.Fatalf("exp.Consequence.Statements[0] not an *ast.ExpressionStatement. got=%T", exp.Consequence.Statements[0])
+	}
+
+	if !testIdentifier(t, consequence.Expression, "x") {
+		return
+	}
+
+	if exp.Alternative != nil {
+		t.Errorf("exp.Alternative was not nil. got=%+v", exp.Alternative)
+	}
+}
+
+func TestIfElseExpression(t *testing.T) {
+	input := `if (x < y) { x } else { y }`
+
+	l := lexer.NewLexer(input)
+	p := parser.NewParser(l)
+	program := p.ParseProgram()
+	checkParserErrors(t, p)
+
+	if len(program.Statements) != 1 {
+		t.Fatalf("program.Statements does not contain 1 statement. got=%d", len(program.Statements))
+	}
+
+	stmt, ok := program.Statements[0].(*ast.ExpressionStatement)
+	if !ok {
+		t.Fatalf("program.Statements[0] not *ast.ExpressionStatement. got=%T", program.Statements[0])
+	}
+
+	exp, ok := stmt.Expression.(*ast.IfExpression)
+	if !ok {
+		t.Fatalf("stmt.Expression not *ast.IfExpression. got=%T", stmt.Expression)
+	}
+
+	if !testInfixExpression(t, exp.Condition, "x", "<", "y") {
+		return
+	}
+
+	if len(exp.Consequence.Statements) != 1 {
+		t.Errorf("exp.Consequence.Statements is not 1 statements. got=%d", len(exp.Consequence.Statements))
+	}
+
+	consequence, ok := exp.Consequence.Statements[0].(*ast.ExpressionStatement)
+	if !ok {
+		t.Fatalf("exp.Consequence.Statements[0] not an *ast.ExpressionStatement. got=%T", exp.Consequence.Statements[0])
+	}
+
+	if !testIdentifier(t, consequence.Expression, "x") {
+		return
+	}
+
+	alternative, ok := exp.Alternative.Statements[0].(*ast.ExpressionStatement)
+	if !ok {
+		t.Fatalf("exp.alternative.Statements[0] not an *ast.ExpressionStatement. got=%T", exp.Consequence.Statements[0])
+	}
+
+	if !testIdentifier(t, alternative.Expression, "y") {
+		return
+	}
+}