Add for statement

src/com/craftinginterpreters/lox/AstPrinter.java

@@ -52,6 +52,18 @@ class AstPrinter implements Expr.Visitor<String>, Stmt.Visitor<String> {
         return parenthesize("define " + statement.name.lexeme + " = ", statement.initializer);
     }
 
+    @Override
+    public String visitWhileStmt(Stmt.While statement) {
+        StringBuilder builder = new StringBuilder();
+
+        builder.append('(');
+        builder.append(parenthesize("while", statement.condition));
+        builder.append(parenthesize("do", statement.body));
+        builder.append(')');
+
+        return builder.toString();
+    }
+
     @Override
     public String visitPrintStmt(Stmt.Print statement) {
         return parenthesize("print", statement.expression);

src/com/craftinginterpreters/lox/Parser.java

@@ -4,6 +4,7 @@ import static com.craftinginterpreters.lox.TokenType.*;
 
 import java.util.ArrayList;
 import java.util.List;
+import java.util.Arrays;
 
 class Parser {
     private static class ParseError extends RuntimeException {
@@ -43,6 +44,8 @@ class Parser {
     }
 
     private Stmt statement() {
+        if (match(FOR))
+            return forStatement();
         if (match(IF))
             return ifStatement();
         if (match(PRINT))
@@ -55,6 +58,48 @@ class Parser {
         return expressionStatement();
     }
 
+    private Stmt forStatement() {
+        consume(LEFT_PAREN, "Expect '(' after 'for'.");
+
+        Stmt initializer;
+        if (match(SEMICOLON)) {
+            initializer = null;
+        } else if (match(VAR)) {
+            initializer = varDeclaration();
+        } else {
+            initializer = expressionStatement();
+        }
+
+        Expr condition = null;
+        if (!check(SEMICOLON)) {
+            condition = expression();
+        }
+        consume(SEMICOLON, "Expect ';' after loop condition.");
+
+        Expr increment = null;
+        if (!check(RIGHT_PAREN)) {
+            increment = expression();
+        }
+        consume(RIGHT_PAREN, "Expect ')' after for clauses.");
+        Stmt body = statement();
+
+        if (increment != null) {
+            body = new Stmt.Block(
+                                  Arrays.asList(
+                                                body,
+                            new Stmt.Expression(increment)));
+        }
+
+        if (condition == null) condition = new Expr.Literal(true);
+        body = new Stmt.While(condition, body);
+
+        if (initializer != null) {
+            body = new Stmt.Block(Arrays.asList(initializer, body));
+        }
+
+        return body;
+    }
+
     private Stmt ifStatement() {
         consume(LEFT_PAREN, "Expect '(' after 'if'.");
         Expr condition = expression();