Review

class ConcreteType.TypeFilter {
    abstract protected C filter(ConcreteType type);
}

class ConcreteType {
      @Override public ConcreteFunctionType filter(ConcreteType type) {
        return type.isFunction() ? type.toFunction() : null;
      }
}

class ConcreteType {
      @Override public ConcreteInstanceType filter(ConcreteType type) {
        return type.isInstance() ? type.toInstance() : null;
      }
}

class ConcreteType {
      @Override public ConcreteInstanceType filter(ConcreteType type) {
        if (type.isFunction()) {
          return type.toFunction().getInstanceType();
        }
        return null;
      }
}

class ConcreteType {
      @Override public ConcreteInstanceType filter(ConcreteType type) {
        if (type.isInstance()
          && type.toInstance().isFunctionPrototype()) {
          return type.toInstance();
        }
        return null;
      }
}

class ConcreteType {
      @Override public ConcreteFunctionType filter(ConcreteType type) {
        return type.isFunction()
          && type.toFunction().getSuperclassType() != null
          ? type.toFunction().getSuperclassType() : null;
      }
}

class ConcreteType {
      @Override public StaticSlot<ConcreteType> filter(ConcreteType type) {
        return type.isFunction()
            && toFunction().getParameterSlot(index) != null
            ? toFunction().getParameterSlot(index) : null;
      }
}

class ConcreteType {
      @Override public StaticSlot<ConcreteType> filter(ConcreteType type) {
        StaticSlot<ConcreteType> slot = null;
        if (type.isInstance()) {
          slot = type.toInstance().getPropertySlot(name);
        }
        return slot;
      }

}