Add conditional execution

.task/checksum/astgen

@@ -1 +1 @@
-53f18a0e7ed4ab1114f57c8bbdaf9abd
+98e69ec471348f6ec8e672a1b8c9825d

ast/printer.go

@@ -34,33 +34,45 @@ func (ap *Printer) VisitExpressionStmt(stmt *ExpressionStmt) any {
 }
 
 func (ap *Printer) VisitPrintStmt(stmt *PrintStmt) any {
-	return ap.parenthesize("print", stmt.Expression)
+	return ap.parenthesizeExpr("print", stmt.Expression)
 }
 
-func (ap *Printer) VisitVarStmt(stmt *VarStmt) any {
+func (ap *Printer) VisitIfStmt(stmt *IfStmt) any {
-	return ap.parenthesize("var", &LiteralExpr{stmt.Name}, stmt.Initializer)
+	str := "(if"
-}
 
-func (ap *Printer) VisitBlockStmt(stmt *BlockStmt) any {
+	if stmt.Condition != nil {
-	str := "(block\n"
+		str += " " + stmt.Condition.Accept(ap).(string)
+	}
 
-	for _, s := range stmt.Statements {
+	if stmt.ThenBranch != nil {
-		str += " " + s.Accept(ap).(string) + "\n"
+		str += " " + ap.bracesizeStmt(stmt.ThenBranch)
+	}
+
+	if stmt.ElseBranch != nil {
+		str += " else " + ap.bracesizeStmt(stmt.ElseBranch)
 	}
 
 	return str + ")"
 }
 
+func (ap *Printer) VisitVarStmt(stmt *VarStmt) any {
+	return ap.parenthesizeExpr("var", &LiteralExpr{stmt.Name}, stmt.Initializer)
+}
+
+func (ap *Printer) VisitBlockStmt(stmt *BlockStmt) any {
+	return ap.bracesizeStmt(stmt.Statements...)
+}
+
 func (ap *Printer) VisitVariableExpr(expr *VariableExpr) any {
 	return expr.Name.Lexeme
 }
 
 func (ap *Printer) VisitBinaryExpr(expr *BinaryExpr) any {
-	return ap.parenthesize(expr.Operator.Lexeme, expr.Left, expr.Right)
+	return ap.parenthesizeExpr(expr.Operator.Lexeme, expr.Left, expr.Right)
 }
 
 func (ap *Printer) VisitGroupingExpr(expr *GroupingExpr) any {
-	return ap.parenthesize("group", expr.Expression)
+	return ap.parenthesizeExpr("group", expr.Expression)
 }
 
 func (ap *Printer) VisitLiteralExpr(expr *LiteralExpr) any {
@@ -71,7 +83,7 @@ func (ap *Printer) VisitLiteralExpr(expr *LiteralExpr) any {
 }
 
 func (ap *Printer) VisitUnaryExpr(expr *UnaryExpr) any {
-	return ap.parenthesize(expr.Operator.Lexeme, expr.Right)
+	return ap.parenthesizeExpr(expr.Operator.Lexeme, expr.Right)
 }
 
 func (ap *Printer) VisitErrorExpr(expr *ErrorExpr) any {
@@ -79,10 +91,10 @@ func (ap *Printer) VisitErrorExpr(expr *ErrorExpr) any {
 }
 
 func (ap *Printer) VisitAssignExpr(expr *AssignExpr) any {
-	return ap.parenthesize("=", &VariableExpr{expr.Name}, expr.Value)
+	return ap.parenthesizeExpr("=", &VariableExpr{expr.Name}, expr.Value)
 }
 
-func (ap *Printer) parenthesize(name string, exprs ...Expr) string {
+func (ap *Printer) parenthesizeExpr(name string, exprs ...Expr) string {
 	str := "(" + name
 
 	for _, expr := range exprs {
@@ -94,3 +106,13 @@ func (ap *Printer) parenthesize(name string, exprs ...Expr) string {
 
 	return str + ")"
 }
+
+func (ap *Printer) bracesizeStmt(stmts ...Stmt) string {
+	str := "{\n"
+
+	for _, s := range stmts {
+		str += " " + s.Accept(ap).(string) + "\n"
+	}
+
+	return str + "}"
+}

ast/printer_test.go

@@ -122,7 +122,26 @@ func TestPrintBlockStmt(t *testing.T) {
 
 	printer := NewPrinter()
 	result := printer.PrintStmts([]Stmt{stmt})
-	expected := "(block\n 1\n 2\n)\n"
+	expected := "{\n 1\n 2\n}\n"
+	if result != expected {
+		t.Errorf("expected %v, got %v", expected, result)
+	}
+}
+
+func TestPrintIfStmt(t *testing.T) {
+	stmt := &IfStmt{
+		Condition: &LiteralExpr{Value: true},
+		ThenBranch: &ExpressionStmt{
+			Expression: &LiteralExpr{Value: 1},
+		},
+		ElseBranch: &ExpressionStmt{
+			Expression: &LiteralExpr{Value: 2},
+		},
+	}
+
+	printer := NewPrinter()
+	result := printer.PrintStmts([]Stmt{stmt})
+	expected := "(if true {\n 1\n} else {\n 2\n})\n"
 	if result != expected {
 		t.Errorf("expected %v, got %v", expected, result)
 	}

ast/stmt.go

@@ -6,6 +6,7 @@ type StmtVisitor[T any] interface {
 	VisitErrorStmt(es *ErrorStmt) T
 	VisitBlockStmt(bs *BlockStmt) T
 	VisitExpressionStmt(es *ExpressionStmt) T
+	VisitIfStmt(is *IfStmt) T
 	VisitPrintStmt(ps *PrintStmt) T
 	VisitVarStmt(vs *VarStmt) T
 }
@@ -38,6 +39,16 @@ func (es *ExpressionStmt) Accept(v StmtVisitor[any]) any {
 	return v.VisitExpressionStmt(es)
 }
 
+type IfStmt struct {
+	Condition Expr
+	ThenBranch Stmt
+	ElseBranch Stmt
+}
+
+func (is *IfStmt) Accept(v StmtVisitor[any]) any {
+	return v.VisitIfStmt(is)
+}
+
 type PrintStmt struct {
 	Expression Expr
 }

cmd/astgen/main.go

@@ -35,6 +35,7 @@ func main() {
 		"Error        : Value string",
 		"Block        : Statements []Stmt",
 		"Expression   : Expression Expr",
+		"If           : Condition Expr, ThenBranch Stmt, ElseBranch Stmt",
 		"Print        : Expression Expr",
 		"Var          : Name token.Token, Initializer Expr",
 	})

interpreter/interpreter.go

@@ -39,6 +39,17 @@ func (i *Interpreter) VisitExpressionStmt(es *ast.ExpressionStmt) any {
 	return nil
 }
 
+// VisitIfStmt visits an if statement.
+func (i *Interpreter) VisitIfStmt(is *ast.IfStmt) any {
+	if isTruthy(i.evaluate(is.Condition)) {
+		i.execute(is.ThenBranch)
+	} else if is.ElseBranch != nil {
+		i.execute(is.ElseBranch)
+	}
+
+	return nil
+}
+
 // VisitPrintStmt visits a print statement.
 func (i *Interpreter) VisitPrintStmt(ps *ast.PrintStmt) any {
 	value := i.evaluate(ps.Expression)

interpreter/interpreter_test.go

@@ -455,3 +455,86 @@ func TestInterpretBlockStatement(t *testing.T) {
 		t.Errorf("run() = %v; want %v", out, expected)
 	}
 }
+
+func TestInterpretIfStatement(t *testing.T) {
+	old := os.Stdout // keep backup of the real stdout
+	r, w, err := os.Pipe()
+	if err != nil {
+		t.Fatal(err)
+	}
+	os.Stdout = w
+
+	outC := make(chan string)
+	// copy the output in a separate goroutine so printing can't block indefinitely
+	go func() {
+		var buf bytes.Buffer
+		io.Copy(&buf, r)
+		outC <- buf.String()
+	}()
+
+	// begin unit test
+	i := New(errors.NewMockErrorLogger())
+	ifStmt := &ast.IfStmt{
+		Condition: &ast.LiteralExpr{Value: true},
+		ThenBranch: &ast.PrintStmt{
+			Expression: &ast.LiteralExpr{Value: 42.0},
+		},
+	}
+
+	i.VisitIfStmt(ifStmt)
+	// end unit test
+
+	// back to normal state
+	w.Close()
+	os.Stdout = old // restoring the real stdout
+	out := <-outC
+
+	// reading our temp stdout
+	expected := "42\n"
+	if out != expected {
+		t.Errorf("run() = %v; want %v", out, expected)
+	}
+}
+
+func TestInterpretIfStatementElseBranch(t *testing.T) {
+	old := os.Stdout // keep backup of the real stdout
+	r, w, err := os.Pipe()
+	if err != nil {
+		t.Fatal(err)
+	}
+	os.Stdout = w
+
+	outC := make(chan string)
+	// copy the output in a separate goroutine so printing can't block indefinitely
+	go func() {
+		var buf bytes.Buffer
+		io.Copy(&buf, r)
+		outC <- buf.String()
+	}()
+
+	// begin unit test
+	i := New(errors.NewMockErrorLogger())
+	ifStmt := &ast.IfStmt{
+		Condition: &ast.LiteralExpr{Value: false},
+		ThenBranch: &ast.PrintStmt{
+			Expression: &ast.LiteralExpr{Value: 42.0},
+		},
+		ElseBranch: &ast.PrintStmt{
+			Expression: &ast.LiteralExpr{Value: 24.0},
+		},
+	}
+
+	i.VisitIfStmt(ifStmt)
+	// end unit test
+
+	// back to normal state
+	w.Close()
+	os.Stdout = old // restoring the real stdout
+	out := <-outC
+
+	// reading our temp stdout
+	expected := "24\n"
+	if out != expected {
+		t.Errorf("run() = %v; want %v", out, expected)
+	}
+}

parser/parser.go

@@ -83,8 +83,11 @@ func (p *Parser) varDeclaration() ast.Stmt {
 	return &ast.VarStmt{Name: name, Initializer: initializer}
 }
 
-// statement → exprStmt | printStmt | block ;
+// statement → exprStmt | ifStmt | printStmt | block ;
 func (p *Parser) statement() ast.Stmt {
+	if p.match(token.IF) {
+		return p.ifStatement()
+	}
 	if p.match(token.PRINT) {
 		return p.printStatement()
 	}
@@ -95,6 +98,40 @@ func (p *Parser) statement() ast.Stmt {
 	return p.expressionStatement()
 }
 
+// ifStmt → "if" "(" expression ")" statement ( "else" statement )? ;
+func (p *Parser) ifStatement() ast.Stmt {
+	err := p.consume(token.LEFT_PAREN, "Expect '(' after 'if'.")
+	if err != nil {
+		return p.fromErrorExpr(err)
+	}
+
+	condition := p.expression()
+
+	err = p.consume(token.RIGHT_PAREN, "Expect ')' after if condition.")
+	if err != nil {
+		return p.fromErrorExpr(err)
+	}
+
+	thenBranch := p.statement()
+	var elseBranch ast.Stmt
+	if p.match(token.ELSE) {
+		elseBranch = p.statement()
+	}
+
+	return &ast.IfStmt{Condition: condition, ThenBranch: thenBranch, ElseBranch: elseBranch}
+}
+
+// printStmt → "print" expression ";" ;
+func (p *Parser) printStatement() ast.Stmt {
+	expr := p.expression()
+	err := p.consume(token.SEMICOLON, "Expect ';' after value.")
+	if err != nil {
+		return p.fromErrorExpr(err)
+	}
+
+	return &ast.PrintStmt{Expression: expr}
+}
+
 // block → "{" declaration* "}" ;
 func (p *Parser) blockStatement() ast.Stmt {
 	statements := []ast.Stmt{}
@@ -122,17 +159,6 @@ func (p *Parser) expressionStatement() ast.Stmt {
 	return &ast.ExpressionStmt{Expression: expr}
 }
 
-// printStmt → "print" expression ";" ;
-func (p *Parser) printStatement() ast.Stmt {
-	expr := p.expression()
-	err := p.consume(token.SEMICOLON, "Expect ';' after value.")
-	if err != nil {
-		return p.fromErrorExpr(err)
-	}
-
-	return &ast.PrintStmt{Expression: expr}
-}
-
 // expression → assignment ;
 func (p *Parser) expression() ast.Expr {
 	return p.assignment()

parser/parser_test.go

@@ -278,7 +278,89 @@ func TestParseBlockStatement(t *testing.T) {
 				{Type: token.RIGHT_BRACE, Lexeme: "}"},
 				{Type: token.EOF},
 			},
-			expected: "(block\n (var foo 42)\n)\n",
+			expected: "{\n (var foo 42)\n}\n",
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			parser := New(tt.tokens, errors.NewMockErrorLogger())
+			stmts := parser.Parse()
+			if len(stmts) != 1 {
+				t.Fatalf("expected 1 statement, got %d", len(stmts))
+			}
+
+			ap := ast.NewPrinter()
+			s := ap.PrintStmts(stmts)
+			if s != tt.expected {
+				t.Errorf("expected %v, got %v", tt.expected, s)
+			}
+		})
+	}
+}
+
+func TestParseIfStatement(t *testing.T) {
+	tests := []struct {
+		name     string
+		tokens   []token.Token
+		expected string
+	}{
+		{
+			name: "simple if statement",
+			tokens: []token.Token{
+				{Type: token.IF, Lexeme: "if"},
+				{Type: token.LEFT_PAREN, Lexeme: "("},
+				{Type: token.NUMBER, Lexeme: "42", Literal: 42},
+				{Type: token.RIGHT_PAREN, Lexeme: ")"},
+				{Type: token.PRINT, Lexeme: "print"},
+				{Type: token.NUMBER, Lexeme: "42", Literal: 42},
+				{Type: token.SEMICOLON, Lexeme: ";"},
+				{Type: token.EOF},
+			},
+			expected: "(if 42 {\n (print 42)\n})\n",
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			parser := New(tt.tokens, errors.NewMockErrorLogger())
+			stmts := parser.Parse()
+			if len(stmts) != 1 {
+				t.Fatalf("expected 1 statement, got %d", len(stmts))
+			}
+
+			ap := ast.NewPrinter()
+			s := ap.PrintStmts(stmts)
+			if s != tt.expected {
+				t.Errorf("expected %v, got %v", tt.expected, s)
+			}
+		})
+	}
+}
+
+func TestParseIfElseStatement(t *testing.T) {
+	tests := []struct {
+		name     string
+		tokens   []token.Token
+		expected string
+	}{
+		{
+			name: "simple if else statement",
+			tokens: []token.Token{
+				{Type: token.IF, Lexeme: "if"},
+				{Type: token.LEFT_PAREN, Lexeme: "("},
+				{Type: token.NUMBER, Lexeme: "42", Literal: 42},
+				{Type: token.RIGHT_PAREN, Lexeme: ")"},
+				{Type: token.PRINT, Lexeme: "print"},
+				{Type: token.NUMBER, Lexeme: "42", Literal: 42},
+				{Type: token.SEMICOLON, Lexeme: ";"},
+				{Type: token.ELSE, Lexeme: "else"},
+				{Type: token.PRINT, Lexeme: "print"},
+				{Type: token.NUMBER, Lexeme: "24", Literal: 24},
+				{Type: token.SEMICOLON, Lexeme: ";"},
+				{Type: token.EOF},
+			},
+			expected: "(if 42 {\n (print 42)\n} else {\n (print 24)\n})\n",
 		},
 	}