Add return statement

src/com/craftinginterpreters/lox/CMakeLists.txt

@@ -21,4 +21,5 @@ add_jar(Lox
   Environment.java
   LoxCallable.java
   LoxFunction.java
+  Return.java
   ENTRY_POINT com/craftinginterpreters/lox/Lox)

src/com/craftinginterpreters/lox/Interpreter.java

@@ -10,15 +10,19 @@ class Interpreter implements Expr.Visitor<Object>, Stmt.Visitor<Void> {
     Interpreter() {
         globals.define("clock", new LoxCallable() {
             @Override
-                public int arity() { return 0; }
-
-                @Override
-                public Object call(Interpreter interpreter, List<Object> arguments) {
-                    return (double)System.currentTimeMillis() / 1000.0;
+            public int arity() {
+                return 0;
             }
 
             @Override
-                public String toString() { return "<native fn>"; }
+            public Object call(Interpreter interpreter, List<Object> arguments) {
+                return (double) System.currentTimeMillis() / 1000.0;
+            }
+
+            @Override
+            public String toString() {
+                return "<native fn>";
+            }
         });
     }
 
@@ -172,6 +176,15 @@ class Interpreter implements Expr.Visitor<Object>, Stmt.Visitor<Void> {
         return null;
     }
 
+    @Override
+    public Void visitReturnStmt(Stmt.Return stmt) {
+        Object value = null;
+        if (stmt.value != null)
+            value = evaluate(stmt.value);
+
+        throw new Return(value);
+    }
+
     @Override
     public Void visitVarStmt(Stmt.Var stmt) {
         Object value = null;

src/com/craftinginterpreters/lox/LoxFunction.java

@@ -22,7 +22,11 @@ class LoxFunction implements LoxCallable {
             environment.define(declaration.params.get(i).lexeme, arguments.get(i));
         }
 
+        try {
             interpreter.executeBlock(declaration.body, environment);
+        } catch (Return returnValue) {
+            return returnValue.value;
+        }
         return null;
     }
 

src/com/craftinginterpreters/lox/Parser.java

@@ -52,6 +52,8 @@ class Parser {
             return ifStatement();
         if (match(PRINT))
             return printStatement();
+        if (match(RETURN))
+            return returnStatement();
         if (match(WHILE))
             return whileStatement();
         if (match(LEFT_BRACE))
@@ -120,6 +122,17 @@ class Parser {
         return new Stmt.Print(value);
     }
 
+    private Stmt returnStatement() {
+        Token keyword = previous();
+        Expr value = null;
+        if (!check(SEMICOLON)) {
+            value = expression();
+        }
+
+        consume(SEMICOLON, "Expect ';' after return value.");
+        return new Stmt.Return(keyword, value);
+    }
+
     private Stmt varDeclaration() {
         Token name = consume(IDENTIFIER, "Expect variable name.");
 

src/com/craftinginterpreters/lox/Return.java

@@ -0,0 +1,10 @@
+package com.craftinginterpreters.lox;
+
+class Return extends RuntimeException {
+    final Object value;
+
+    Return(Object value) {
+        super(null, null, false, false);
+        this.value = value;
+    }
+}

src/com/craftinginterpreters/tool/GenerateAst.java

@@ -23,7 +23,8 @@ public class GenerateAst {
                 Arrays.asList("Block : List<Stmt> statements", "Expression : Expr expression",
                         "Function : Token name, List<Token> params, List<Stmt> body",
                         "If : Expr condition, Stmt thenBranch, Stmt elseBranch", "Print : Expr expression",
-                        "Var : Token name, Expr initializer", "While : Expr condition, Stmt body"));
+                        "Return : Token keyword, Expr value", "Var : Token name, Expr initializer",
+                        "While : Expr condition, Stmt body"));
     }
 
     private static void defineAst(String outputDir, String baseName, List<String> types) throws IOException {