added undef value

src/main/java/carpet/script/value/NullValue.java

@@ -32,7 +32,7 @@ public class NullValue extends NumericValue // TODO check nonsingleton code
     {
         return new NullValue();
     }
-    private NullValue() {super(0);}
+    protected NullValue() {super(0);}
 
     @Override
     public boolean equals(final Object o)

src/main/java/carpet/script/value/UndefValue.java

@@ -0,0 +1,140 @@
+package carpet.script.value;
+
+import carpet.script.exception.InternalExpressionException;
+import com.google.gson.JsonElement;
+import net.minecraft.nbt.Tag;
+
+public class UndefValue extends NullValue {
+    public static final UndefValue UNDEF = new UndefValue();
+
+    private RuntimeException getError() {
+        return new InternalExpressionException("variable "+boundVariable+" was used before initialization under 'strict' app config");
+    }
+
+
+    @Override
+    public String getString()
+    {
+        throw getError();
+    }
+
+    @Override
+    public String getPrettyString()
+    {
+        return "undefined";
+    }
+
+    @Override
+    public boolean getBoolean()
+    {
+        throw getError();
+    }
+
+    @Override
+    public Value clone()
+    {
+        return new UndefValue();
+    }
+
+    @Override
+    public boolean equals(final Object o)
+    {
+        throw getError();
+    }
+
+    @Override
+    public Value slice(long fromDesc, Long toDesc) {
+        throw getError();
+    }
+
+    @Override
+    public NumericValue opposite() {
+        throw getError();
+    }
+
+    @Override
+    public int length() {
+        throw getError();
+    }
+
+    @Override
+    public int compareTo(Value o)
+    {
+        throw getError();
+    }
+
+    @Override
+    public Value in(Value value) {
+        throw getError();
+    }
+
+    @Override
+    public String getTypeString()
+    {
+        return "undef";
+    }
+
+    @Override
+    public int hashCode()
+    {
+        throw getError();
+    }
+
+    @Override
+    public Tag toTag(boolean force)
+    {
+        throw getError();
+    }
+
+    @Override
+    public Value split(Value delimiter) {
+        throw getError();
+    }
+
+    @Override
+    public JsonElement toJson()
+    {
+        throw getError();
+    }
+
+    @Override
+    public boolean isNull() {
+        throw getError();
+    }
+
+    @Override
+    public Value add(Value v) {
+        throw getError();
+    }
+
+    @Override
+    public Value subtract(Value v)
+    {
+        throw getError();
+    }
+
+    @Override
+    public Value multiply(Value v)
+    {
+        throw getError();
+    }
+
+    @Override
+    public Value divide(Value v)
+    {
+        throw getError();
+    }
+
+    @Override
+    public double readDoubleNumber()
+    {
+        throw getError();
+    }
+
+    @Override
+    public long readInteger()
+    {
+        throw getError();
+    }
+
+}

src/main/java/carpet/script/value/Value.java

@@ -21,6 +21,7 @@ public abstract class Value implements Comparable<Value>, Cloneable
     public static NumericValue ONE = new NumericValue(1);
 
     public static NullValue NULL = NullValue.NULL;
+    public static UndefValue UNDEF = UndefValue.UNDEF;
 
     public String boundVariable;