diff --git a/src/main/java/com/annimon/ownlang/lib/modules/canvas.java b/src/main/java/com/annimon/ownlang/lib/modules/canvas.java
index 7a9fc05..1f1ca36 100644
--- a/src/main/java/com/annimon/ownlang/lib/modules/canvas.java
+++ b/src/main/java/com/annimon/ownlang/lib/modules/canvas.java
@@ -32,12 +32,12 @@ public final class canvas implements Module {
     private static ArrayValue mouseHover;
 
     public static void initConstants() {
-        Variables.set("VK_UP", NumberValue.of(KeyEvent.VK_UP));
-        Variables.set("VK_DOWN", NumberValue.of(KeyEvent.VK_DOWN));
-        Variables.set("VK_LEFT", NumberValue.of(KeyEvent.VK_LEFT));
-        Variables.set("VK_RIGHT", NumberValue.of(KeyEvent.VK_RIGHT));
-        Variables.set("VK_FIRE", NumberValue.of(KeyEvent.VK_ENTER));
-        Variables.set("VK_ESCAPE", NumberValue.of(KeyEvent.VK_ESCAPE));
+        Variables.define("VK_UP", NumberValue.of(KeyEvent.VK_UP));
+        Variables.define("VK_DOWN", NumberValue.of(KeyEvent.VK_DOWN));
+        Variables.define("VK_LEFT", NumberValue.of(KeyEvent.VK_LEFT));
+        Variables.define("VK_RIGHT", NumberValue.of(KeyEvent.VK_RIGHT));
+        Variables.define("VK_FIRE", NumberValue.of(KeyEvent.VK_ENTER));
+        Variables.define("VK_ESCAPE", NumberValue.of(KeyEvent.VK_ESCAPE));
     }
 
     @Override
diff --git a/src/main/java/com/annimon/ownlang/lib/modules/canvasfx.java b/src/main/java/com/annimon/ownlang/lib/modules/canvasfx.java
index c175121..c8fb30a 100644
--- a/src/main/java/com/annimon/ownlang/lib/modules/canvasfx.java
+++ b/src/main/java/com/annimon/ownlang/lib/modules/canvasfx.java
@@ -101,67 +101,67 @@ public final class canvasfx implements Module {
         colors.put(new StringValue("rgb"), new FunctionValue(new rgbColor()));
         colors.put(new StringValue("hsb"), new FunctionValue(new hsbColor()));
         colors.put(new StringValue("web"), new FunctionValue(new webColor()));
-        Variables.set("Color", new MapValue(colors));
+        Variables.define("Color", new MapValue(colors));
 
         final MapValue arcType = new MapValue(ArcType.values().length);
         for (ArcType value : ArcType.values()) {
             arcType.set(new StringValue(value.name()), NumberValue.of(value.ordinal()));
         }
-        Variables.set("ArcType", arcType);
+        Variables.define("ArcType", arcType);
 
         final MapValue fillRule = new MapValue(FillRule.values().length);
         for (FillRule value : FillRule.values()) {
             fillRule.set(new StringValue(value.name()), NumberValue.of(value.ordinal()));
         }
-        Variables.set("FillRule", fillRule);
+        Variables.define("FillRule", fillRule);
 
         final MapValue blendMode = new MapValue(BlendMode.values().length);
         for (BlendMode value : BlendMode.values()) {
             blendMode.set(new StringValue(value.name()), NumberValue.of(value.ordinal()));
         }
-        Variables.set("BlendMode", blendMode);
+        Variables.define("BlendMode", blendMode);
 
         final MapValue lineCap = new MapValue(StrokeLineCap.values().length);
         for (StrokeLineCap value : StrokeLineCap.values()) {
             lineCap.set(new StringValue(value.name()), NumberValue.of(value.ordinal()));
         }
-        Variables.set("StrokeLineCap", lineCap);
+        Variables.define("StrokeLineCap", lineCap);
 
         final MapValue lineJoin = new MapValue(StrokeLineJoin.values().length);
         for (StrokeLineJoin value : StrokeLineJoin.values()) {
             lineJoin.set(new StringValue(value.name()), NumberValue.of(value.ordinal()));
         }
-        Variables.set("StrokeLineJoin", lineJoin);
+        Variables.define("StrokeLineJoin", lineJoin);
 
         final MapValue textAlignment = new MapValue(TextAlignment.values().length);
         for (TextAlignment value : TextAlignment.values()) {
             textAlignment.set(new StringValue(value.name()), NumberValue.of(value.ordinal()));
         }
-        Variables.set("TextAlignment", textAlignment);
+        Variables.define("TextAlignment", textAlignment);
 
         final MapValue vPos = new MapValue(VPos.values().length);
         for (VPos value : VPos.values()) {
             vPos.set(new StringValue(value.name()), NumberValue.of(value.ordinal()));
         }
-        Variables.set("VPos", vPos);
+        Variables.define("VPos", vPos);
 
         final MapValue events = new MapValue(Events.values().length);
         for (Events value : Events.values()) {
             events.set(new StringValue(value.name()), NumberValue.of(value.ordinal()));
         }
-        Variables.set("Events", events);
+        Variables.define("Events", events);
 
         final MapValue mouseButton = new MapValue(MouseButton.values().length);
         for (MouseButton value : MouseButton.values()) {
             mouseButton.set(new StringValue(value.name()), NumberValue.of(value.ordinal()));
         }
-        Variables.set("MouseButton", mouseButton);
+        Variables.define("MouseButton", mouseButton);
 
         final MapValue keyCodes = new MapValue(KeyCode.values().length);
         for (KeyCode value : KeyCode.values()) {
             keyCodes.set(new StringValue(value.name()), NumberValue.of(value.ordinal()));
         }
-        Variables.set("KeyCode", keyCodes);
+        Variables.define("KeyCode", keyCodes);
     }
 
     @Override
diff --git a/src/main/java/com/annimon/ownlang/lib/modules/date.java b/src/main/java/com/annimon/ownlang/lib/modules/date.java
index 73376aa..a3de9f8 100644
--- a/src/main/java/com/annimon/ownlang/lib/modules/date.java
+++ b/src/main/java/com/annimon/ownlang/lib/modules/date.java
@@ -29,10 +29,10 @@ public final class date implements Module {
             MILLISECOND = new StringValue("millisecond");
 
     public static void initConstants() {
-        Variables.set("STYLE_FULL", NumberValue.of(DateFormat.FULL));
-        Variables.set("STYLE_LONG", NumberValue.of(DateFormat.LONG));
-        Variables.set("STYLE_MEDIUM", NumberValue.of(DateFormat.MEDIUM));
-        Variables.set("STYLE_SHORT", NumberValue.of(DateFormat.SHORT));
+        Variables.define("STYLE_FULL", NumberValue.of(DateFormat.FULL));
+        Variables.define("STYLE_LONG", NumberValue.of(DateFormat.LONG));
+        Variables.define("STYLE_MEDIUM", NumberValue.of(DateFormat.MEDIUM));
+        Variables.define("STYLE_SHORT", NumberValue.of(DateFormat.SHORT));
     }
 
     @Override
diff --git a/src/main/java/com/annimon/ownlang/lib/modules/files.java b/src/main/java/com/annimon/ownlang/lib/modules/files.java
index 04c1010..f65d848 100644
--- a/src/main/java/com/annimon/ownlang/lib/modules/files.java
+++ b/src/main/java/com/annimon/ownlang/lib/modules/files.java
@@ -27,13 +27,13 @@ public final class files implements Module {
     private static Map<Integer, FileInfo> files;
 
     public static void initConstants() {
+        Variables.define("FILES_COMPARATOR", new FunctionValue(new filesComparatorFunction()));
     }
 
     @Override
     public void init() {
         files = new HashMap<>();
         initConstants();
-        Variables.set("FILES_COMPARATOR", new FunctionValue(new filesComparatorFunction()));
         
         Functions.set("fopen", new fopen());
         Functions.set("flush", new flush());
diff --git a/src/main/java/com/annimon/ownlang/lib/modules/functional.java b/src/main/java/com/annimon/ownlang/lib/modules/functional.java
index f3db9c8..8bedd85 100644
--- a/src/main/java/com/annimon/ownlang/lib/modules/functional.java
+++ b/src/main/java/com/annimon/ownlang/lib/modules/functional.java
@@ -12,7 +12,7 @@ import com.annimon.ownlang.lib.modules.functions.*;
 public final class functional implements Module {
 
     public static void initConstants() {
-        Variables.set("IDENTITY", new FunctionValue(args -> args[0]));
+        Variables.define("IDENTITY", new FunctionValue(args -> args[0]));
     }
 
     @Override
diff --git a/src/main/java/com/annimon/ownlang/lib/modules/math.java b/src/main/java/com/annimon/ownlang/lib/modules/math.java
index 33b48cb..826a624 100644
--- a/src/main/java/com/annimon/ownlang/lib/modules/math.java
+++ b/src/main/java/com/annimon/ownlang/lib/modules/math.java
@@ -17,8 +17,8 @@ public final class math implements Module {
     private static final DoubleFunction<NumberValue> doubleToNumber = NumberValue::of;
 
     public static void initConstants() {
-        Variables.set("PI", NumberValue.of(Math.PI));
-        Variables.set("E", NumberValue.of(Math.E));
+        Variables.define("PI", NumberValue.of(Math.PI));
+        Variables.define("E", NumberValue.of(Math.E));
     }
 
     @Override
diff --git a/src/main/java/com/annimon/ownlang/lib/modules/robot.java b/src/main/java/com/annimon/ownlang/lib/modules/robot.java
index 7e33bbc..10990c6 100644
--- a/src/main/java/com/annimon/ownlang/lib/modules/robot.java
+++ b/src/main/java/com/annimon/ownlang/lib/modules/robot.java
@@ -32,15 +32,15 @@ public final class robot implements Module {
     private static Robot awtRobot;
 
     public static void initConstants() {
-        Variables.set("VK_DOWN", NumberValue.of(KeyEvent.VK_DOWN));
-        Variables.set("VK_LEFT", NumberValue.of(KeyEvent.VK_LEFT));
-        Variables.set("VK_RIGHT", NumberValue.of(KeyEvent.VK_RIGHT));
-        Variables.set("VK_FIRE", NumberValue.of(KeyEvent.VK_ENTER));
-        Variables.set("VK_ESCAPE", NumberValue.of(KeyEvent.VK_ESCAPE));
+        Variables.define("VK_DOWN", NumberValue.of(KeyEvent.VK_DOWN));
+        Variables.define("VK_LEFT", NumberValue.of(KeyEvent.VK_LEFT));
+        Variables.define("VK_RIGHT", NumberValue.of(KeyEvent.VK_RIGHT));
+        Variables.define("VK_FIRE", NumberValue.of(KeyEvent.VK_ENTER));
+        Variables.define("VK_ESCAPE", NumberValue.of(KeyEvent.VK_ESCAPE));
 
-        Variables.set("BUTTON1", NumberValue.of(InputEvent.BUTTON1_MASK));
-        Variables.set("BUTTON2", NumberValue.of(InputEvent.BUTTON2_MASK));
-        Variables.set("BUTTON3", NumberValue.of(InputEvent.BUTTON3_MASK));
+        Variables.define("BUTTON1", NumberValue.of(InputEvent.BUTTON1_MASK));
+        Variables.define("BUTTON2", NumberValue.of(InputEvent.BUTTON2_MASK));
+        Variables.define("BUTTON3", NumberValue.of(InputEvent.BUTTON3_MASK));
     }
 
     @Override
diff --git a/src/main/java/com/annimon/ownlang/lib/modules/std.java b/src/main/java/com/annimon/ownlang/lib/modules/std.java
index 21cb0df..00be346 100644
--- a/src/main/java/com/annimon/ownlang/lib/modules/std.java
+++ b/src/main/java/com/annimon/ownlang/lib/modules/std.java
@@ -13,7 +13,7 @@ import com.annimon.ownlang.lib.modules.functions.*;
 public final class std implements Module {
 
     public static void initConstants() {
-        Variables.set("ARGS", ArrayValue.of(Main.getOwnlangArgs()));
+        Variables.define("ARGS", ArrayValue.of(Main.getOwnlangArgs()));
     }
 
     @Override
diff --git a/src/main/java/com/annimon/ownlang/lib/modules/types.java b/src/main/java/com/annimon/ownlang/lib/modules/types.java
index 17c3ec7..f207d7d 100644
--- a/src/main/java/com/annimon/ownlang/lib/modules/types.java
+++ b/src/main/java/com/annimon/ownlang/lib/modules/types.java
@@ -11,12 +11,12 @@ import com.annimon.ownlang.lib.*;
 public final class types implements Module {
 
     public static void initConstants() {
-        Variables.set("OBJECT", NumberValue.of(Types.OBJECT));
-        Variables.set("NUMBER", NumberValue.of(Types.NUMBER));
-        Variables.set("STRING", NumberValue.of(Types.STRING));
-        Variables.set("ARRAY", NumberValue.of(Types.ARRAY));
-        Variables.set("MAP", NumberValue.of(Types.MAP));
-        Variables.set("FUNCTION", NumberValue.of(Types.FUNCTION));
+        Variables.define("OBJECT", NumberValue.of(Types.OBJECT));
+        Variables.define("NUMBER", NumberValue.of(Types.NUMBER));
+        Variables.define("STRING", NumberValue.of(Types.STRING));
+        Variables.define("ARRAY", NumberValue.of(Types.ARRAY));
+        Variables.define("MAP", NumberValue.of(Types.MAP));
+        Variables.define("FUNCTION", NumberValue.of(Types.FUNCTION));
     }
 
     @Override