From 0dbf155da52c5db1cab2688b04c87386a55c5d5b Mon Sep 17 00:00:00 2001
From: Tom Willemse <tom@ryuslash.org>
Date: Mon, 8 Feb 2021 17:42:18 -0800
Subject: [PATCH] Add return statement

---
 .../craftinginterpreters/lox/CMakeLists.txt   |  1 +
 .../craftinginterpreters/lox/Interpreter.java | 31 +++++++++++++------
 .../craftinginterpreters/lox/LoxFunction.java |  6 +++-
 src/com/craftinginterpreters/lox/Parser.java  | 13 ++++++++
 src/com/craftinginterpreters/lox/Return.java  | 10 ++++++
 .../tool/GenerateAst.java                     |  3 +-
 6 files changed, 53 insertions(+), 11 deletions(-)
 create mode 100644 src/com/craftinginterpreters/lox/Return.java

diff --git a/src/com/craftinginterpreters/lox/CMakeLists.txt b/src/com/craftinginterpreters/lox/CMakeLists.txt
index a068037..fa93588 100644
--- a/src/com/craftinginterpreters/lox/CMakeLists.txt
+++ b/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)
diff --git a/src/com/craftinginterpreters/lox/Interpreter.java b/src/com/craftinginterpreters/lox/Interpreter.java
index 407c537..4549e11 100644
--- a/src/com/craftinginterpreters/lox/Interpreter.java
+++ b/src/com/craftinginterpreters/lox/Interpreter.java
@@ -9,17 +9,21 @@ class Interpreter implements Expr.Visitor<Object>, Stmt.Visitor<Void> {
 
     Interpreter() {
         globals.define("clock", new LoxCallable() {
-                @Override
-                public int arity() { return 0; }
+            @Override
+            public int arity() {
+                return 0;
+            }
 
-                @Override
-                public Object call(Interpreter interpreter, List<Object> arguments) {
-                    return (double)System.currentTimeMillis() / 1000.0;
-                }
+            @Override
+            public Object call(Interpreter interpreter, List<Object> arguments) {
+                return (double) System.currentTimeMillis() / 1000.0;
+            }
 
-                @Override
-                public String toString() { return "<native fn>"; }
-            });
+            @Override
+            public String toString() {
+                return "<native fn>";
+            }
+        });
     }
 
     @Override
@@ -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;
diff --git a/src/com/craftinginterpreters/lox/LoxFunction.java b/src/com/craftinginterpreters/lox/LoxFunction.java
index d2e367b..0f7ebd4 100644
--- a/src/com/craftinginterpreters/lox/LoxFunction.java
+++ b/src/com/craftinginterpreters/lox/LoxFunction.java
@@ -22,7 +22,11 @@ class LoxFunction implements LoxCallable {
             environment.define(declaration.params.get(i).lexeme, arguments.get(i));
         }
 
-        interpreter.executeBlock(declaration.body, environment);
+        try {
+            interpreter.executeBlock(declaration.body, environment);
+        } catch (Return returnValue) {
+            return returnValue.value;
+        }
         return null;
     }
 
diff --git a/src/com/craftinginterpreters/lox/Parser.java b/src/com/craftinginterpreters/lox/Parser.java
index c427bc4..528326c 100644
--- a/src/com/craftinginterpreters/lox/Parser.java
+++ b/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.");
 
diff --git a/src/com/craftinginterpreters/lox/Return.java b/src/com/craftinginterpreters/lox/Return.java
new file mode 100644
index 0000000..5bd62d0
--- /dev/null
+++ b/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;
+    }
+}
diff --git a/src/com/craftinginterpreters/tool/GenerateAst.java b/src/com/craftinginterpreters/tool/GenerateAst.java
index 92c7967..2bf442b 100644
--- a/src/com/craftinginterpreters/tool/GenerateAst.java
+++ b/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 {