[jlox] Add branching

jlox/lox/src/main/java/xyz/ctsk/lox/AstPrinter.java

@@ -43,6 +43,15 @@ public class AstPrinter {
             return expr.value().toString();
         }
 
+        @Override
+        public String visitLogicalExpr(Expr.Logical expr) {
+            var left = expr.left().accept(this);
+            var op = expr.operator().lexeme();
+            var right = expr.right().accept(this);
+
+            return reverse ? wrap(left, right, op) : wrap(op, left, right);
+        }
+
         @Override
         public String visitUnaryExpr(Expr.Unary expr) {
             var op = expr.operator().lexeme();
@@ -91,6 +100,14 @@ public class AstPrinter {
             return Interpreter.stringify(expr.value());
         }
 
+        @Override
+        public String visitLogicalExpr(Expr.Logical expr) {
+            return String.join(" ",
+                    expr.left().accept(this),
+                    expr.operator().lexeme(),
+                    expr.right().accept(this));
+        }
+
         @Override
         public String visitUnaryExpr(Expr.Unary expr) {
             return expr.operator().lexeme() + expr.right().accept(this);

jlox/lox/src/main/java/xyz/ctsk/lox/Interpreter.java

@@ -55,6 +55,16 @@ public class Interpreter implements Expr.Visitor<Object>, Stmt.Visitor<Void> {
         return null;
     }
 
+    @Override
+    public Void visitIfStmt(Stmt.If stmt) {
+        if (isTruthy(stmt.condition())) {
+            execute(stmt.thenBranch());
+        } else if (stmt.elseBranch() != null){
+            execute(stmt.elseBranch());
+        }
+        return null;
+    }
+
     @Override
     public Void visitPrintStmt(Stmt.Print stmt) {
         Object value = evaluate(stmt.expression());
@@ -121,6 +131,19 @@ public class Interpreter implements Expr.Visitor<Object>, Stmt.Visitor<Void> {
         return expr.value();
     }
 
+    @Override
+    public Object visitLogicalExpr(Expr.Logical expr) {
+        var leftVal = evaluate(expr.left());
+
+        if (expr.operator().type() == TokenType.OR) {
+            if (isTruthy(leftVal)) return leftVal;
+        } else {
+            if (!isTruthy(leftVal)) return leftVal;
+        }
+
+        return evaluate(expr.right());
+    }
+
     @Override
     public Object visitUnaryExpr(Expr.Unary expr) {
         var right = evaluate(expr.right());

jlox/lox/src/main/java/xyz/ctsk/lox/Parser.java

@@ -15,6 +15,7 @@ import static xyz.ctsk.lox.TokenType.*;
  *                | statement ;
  * varDecl        → "var" IDENTIFIER ( "=" expression )? ";" ;
  * statement      → exprStmt
+ *                | ifStmt
  *                | printStmt
  *                | block ;
  * block          → "{" declaration* "}" ;
@@ -23,7 +24,9 @@ import static xyz.ctsk.lox.TokenType.*;
  * expression     → equality ;
  * expression     → assignment ;
  * assignment     → IDENTIFIER "=" assignment
- *                | equality ;
+ *                | logic_or ;
+ * logic_or       → logic_and ( "or" logic_and )* ;
+ * logic_and      → equality ( "and" equality )* ;
  * equality       → comparison ( ( "!=" | "==" ) comparison )* ;
  * comparison     → term ( ( ">" | ">=" | "<" | "<=" ) term )* ;
  * term           → factor ( ( "-" | "+" ) factor )* ;
@@ -76,10 +79,23 @@ public class Parser {
 
 
     private Stmt statement() {
+        if (match(IF)) return ifStatement();
         if (match(PRINT)) return printStatement();
         if (match(LEFT_BRACE)) return new Stmt.Block(blockStatement());
         return expressionStatement();
     }
+
+    private Stmt ifStatement() {
+        consume(LEFT_PAREN, "Expect '(' after 'if'.");
+        Expr condition = expression();
+        consume(RIGHT_PAREN, "Expect ')' after condition.");
+
+        Stmt thenBranch = statement();
+        Stmt elseBranch = match(ELSE) ? statement() : null;
+
+        return new Stmt.If(condition, thenBranch, elseBranch);
+    }
+
     private Stmt printStatement() {
         Expr value = expression();
         consume(SEMICOLON, "Expect ';' after value.");
@@ -108,7 +124,7 @@ public class Parser {
     }
 
     private Expr assignment() {
-        Expr expr = equality();
+        Expr expr = or();
 
         if (match(EQUAL)) {
             Token equals = previous();
@@ -126,6 +142,30 @@ public class Parser {
         return expr;
     }
 
+    private Expr or() {
+        Expr expr = and();
+
+        while (match(OR)) {
+            Token operator = previous();
+            Expr right = and();
+            expr = new Expr.Logical(expr, operator, right);
+        }
+
+        return expr;
+    }
+
+    private Expr and() {
+        Expr expr = equality();
+
+        while (match(AND)) {
+            Token operator = previous();
+            Expr right = equality();
+            expr  = new Expr.Logical(expr, operator, right);
+        }
+
+        return expr;
+    }
+
     private Expr equality() {
         Expr expr = comparison();
 

jlox/lox/src/main/java/xyz/ctsk/lox/package-info.java

@@ -5,6 +5,7 @@
                         @Rule(head = "Binary", body = {"Expr left", "Token operator", "Expr right"}),
                         @Rule(head = "Grouping", body = {"Expr expression"}),
                         @Rule(head = "Literal", body = {"Object value"}),
+                        @Rule(head = "Logical", body = {"Expr left", "Token operator", "Expr right"}),
                         @Rule(head = "Unary", body = {"Token operator", "Expr right"}),
                         @Rule(head = "Variable", body = {"Token name"})
                 }),
@@ -12,6 +13,7 @@
                 rules = {
                         @Rule(head = "Block", body = {"List<Stmt> statements"}),
                         @Rule(head = "Expression", body = {"Expr expression"}),
+                        @Rule(head = "If", body = {"Expr condition", "Stmt thenBranch", "Stmt elseBranch"}),
                         @Rule(head = "Print", body = {"Expr expression"}),
                         @Rule(head = "Var", body = {"Token name", "Expr initializer"})
                 })