[9.2] Add support for conditional execution

src/Interpreter.php

@@ -12,6 +12,7 @@
 use Lox\Expr\Visitor as VisitorExpr;
 use Lox\Stmt\BlockStmt;
 use Lox\Stmt\ExpressionStmt;
+use Lox\Stmt\IfStmt;
 use Lox\Stmt\PrintStmt;
 use Lox\Stmt\Stmt;
 use Lox\Stmt\VarStmt;
@@ -146,6 +147,16 @@ public function visitExpressionStmt(ExpressionStmt $stmt): void
         $this->evaluate($stmt->expression());
     }
 
+    #[\Override]
+    public function visitIfStmt(IfStmt $stmt): void
+    {
+        if ($this->isTruthy($this->evaluate($stmt->condition()))) {
+            $this->execute($stmt->thenBranch());
+        } elseif (null !== $stmt->elseBranch()) {
+            $this->execute($stmt->elseBranch());
+        }
+    }
+
     #[\Override]
     public function visitPrintStmt(PrintStmt $stmt): void
     {

src/Parser.php

@@ -11,6 +11,7 @@
 use Lox\Expr\Variable;
 use Lox\Stmt\BlockStmt;
 use Lox\Stmt\ExpressionStmt;
+use Lox\Stmt\IfStmt;
 use Lox\Stmt\PrintStmt;
 use Lox\Stmt\Stmt;
 use Lox\Stmt\VarStmt;
@@ -66,6 +67,10 @@ private function declaration(): ?Stmt
 
     private function statement(): Stmt
     {
+        if ($this->match(TokenType::IF)) {
+            return $this->ifStatement();
+        }
+
         if ($this->match(TokenType::PRINT)) {
             return $this->printStatement();
         }
@@ -77,6 +82,21 @@ private function statement(): Stmt
         return $this->expressionStatement();
     }
 
+    private function ifStatement(): Stmt
+    {
+        $this->consume(TokenType::LEFT_PAREN, 'Expect "(" after "if".');
+        $condition = $this->expression();
+        $this->consume(TokenType::RIGHT_PAREN, 'Expect ")" after if condition.');
+
+        $thenBranch = $this->statement();
+        $elseBranch = null;
+        if ($this->match(TokenType::ELSE)) {
+            $elseBranch = $this->statement();
+        }
+
+        return new IfStmt($condition, $thenBranch, $elseBranch);
+    }
+
     private function printStatement(): Stmt
     {
         $value = $this->expression();

src/Stmt/IfStmt.php

@@ -0,0 +1,46 @@
+<?php
+
+/**
+ * This file is auto-generated.
+ */
+
+declare(strict_types=1);
+
+namespace Lox\Stmt;
+
+use Lox\Expr\Expr;
+
+final class IfStmt extends Stmt
+{
+    private Expr $condition;
+    private Stmt $thenBranch;
+    private ?Stmt $elseBranch;
+
+    public function __construct(Expr $condition, Stmt $thenBranch, ?Stmt $elseBranch)
+    {
+        $this->condition = $condition;
+        $this->thenBranch = $thenBranch;
+        $this->elseBranch = $elseBranch;
+    }
+
+    public function condition(): Expr
+    {
+        return $this->condition;
+    }
+
+    public function thenBranch(): Stmt
+    {
+        return $this->thenBranch;
+    }
+
+    public function elseBranch(): ?Stmt
+    {
+        return $this->elseBranch;
+    }
+
+    #[\Override]
+    public function accept(Visitor $visitor)
+    {
+        return $visitor->visitIfStmt($this);
+    }
+}

src/Stmt/Visitor.php

@@ -14,6 +14,8 @@ public function visitBlockStmt(BlockStmt $stmt): void;
 
     public function visitExpressionStmt(ExpressionStmt $stmt): void;
 
+    public function visitIfStmt(IfStmt $stmt): void;
+
     public function visitPrintStmt(PrintStmt $stmt): void;
 
     public function visitVarStmt(VarStmt $stmt): void;

tests/lox/block/empty.lox

@@ -1,3 +1,7 @@
 {} // By itself.
 
+// In a statement.
+if (true) {}
+if (false) {} else {}
+
 print "ok"; // expect: ok

tests/lox/if/dangling_else.lox

@@ -0,0 +1,3 @@
+// A dangling else binds to the right-most if.
+if (true) if (false) print "bad"; else print "good"; // expect: good
+if (false) if (true) print "bad"; else print "bad";

tests/lox/if/else.lox

@@ -0,0 +1,6 @@
+// Evaluate the 'else' expression if the condition is false.
+if (true) print "good"; else print "bad"; // expect: good
+if (false) print "bad"; else print "good"; // expect: good
+
+// Allow block body.
+if (false) nil; else { print "block"; } // expect: block

tests/lox/if/if.lox

@@ -0,0 +1,10 @@
+// Evaluate the 'then' expression if the condition is true.
+if (true) print "good"; // expect: good
+if (false) print "bad";
+
+// Allow block body.
+if (true) { print "block"; } // expect: block
+
+// Assignment in if condition.
+var a = false;
+if (a = true) print a; // expect: true

tests/lox/if/truth.lox

@@ -0,0 +1,8 @@
+// False and nil are false.
+if (false) print "bad"; else print "false"; // expect: false
+if (nil) print "bad"; else print "nil"; // expect: nil
+
+// Everything else is true.
+if (true) print true; // expect: true
+if (0) print 0; // expect: 0
+if ("") print "empty"; // expect: empty

tests/lox/if/var_in_else.lox

@@ -0,0 +1,2 @@
+// [line 2] Error at "var": Expect expression.
+if (true) "ok"; else var foo;

tests/lox/if/var_in_then.lox

@@ -0,0 +1,2 @@
+// [line 2] Error at "var": Expect expression.
+if (true) var foo;

tests/lox/variable/unreached_undefined.lox

@@ -0,0 +1,5 @@
+if (false) {
+  print notDefined;
+}
+
+print "ok"; // expect: ok

tools/generate-ast

@@ -22,8 +22,9 @@ defineAst(EXPR_DIR, 'Expr', [
 
 // Stmt classes must have Stmt suffix to be valid
 defineAst(STMT_DIR, 'Stmt', [
-    'BlockStmt      : List<Stmt> statements',
+    'BlockStmt      : list<Stmt> statements',
     'ExpressionStmt : Lox\Expr\Expr expression',
+    'IfStmt         : Lox\Expr\Expr condition, Lox\Stmt\Stmt thenBranch, ?Lox\Stmt\Stmt elseBranch',
     'PrintStmt      : Lox\Expr\Expr expression',
     'VarStmt        : Lox\Token name, ?Lox\Expr\Expr initializer',
 ]);
@@ -107,10 +108,10 @@ function generateType(string $baseName, string $className, array $properties = [
             $type = $parts[0];
             $propertyName = $parts[1];
 
-            if (preg_match('/^List<.*>$/', $type)) {
-                $propertyComment = sprintf('@var %s', lcfirst($type));
-                $constructorComment = sprintf('@param %s $%s', lcfirst($type), $propertyName);
-                $returnComment = sprintf('@return %s', lcfirst($type));
+            if (preg_match('/^list<.*>$/', $type)) {
+                $propertyComment = sprintf('@var %s', $type);
+                $constructorComment = sprintf('@param %s $%s', $type, $propertyName);
+                $returnComment = sprintf('@return %s', $type);
                 $type = 'array';
             }
         } else {