How to change class's metaclass - common-lisp

This happens to me time and again: I define the class and forget that I wanted it funcallable or it is, say, Gtk widget class, thus it's metaclass needs to be stated. Once it is defined, however, SBCL doesn't let me change me the metaclass (even if there is no instance of this class). For example, evaluating
(defclass foo ()
((slot-a)))
and then adding a metaclass and re-evaluating:
(defclass foo ()
((slot-a))
(:metaclass gobject:gobject-class))
results in error:
Cannot CHANGE-CLASS objects into CLASS metaobjects.
[Condition of type SB-PCL::METAOBJECT-INITIALIZATION-VIOLATION]
See also:
The Art of the Metaobject Protocol, CLASS [:initialization]
Unfortunately I don't have a copy of The Art of the Metaobject Protocol to check what it says. For now the only way I could figure out is to restart lisp, which can be quite disruptive.
Since I realise the error soon enough, I don't mind dodging the defined class completely by removing it. Questions:
If I have created instances of the class, is there a way to find them to nullify them and get them GCed?
How to remove the class? Something like fmakunbound for functions.

Unfortunately I don't have a copy of The Art of the Metaobject Protocol to check what it says.
Even though I recommend reading the book, you can find some information online. See for example ENSURE-CLASS-USING-CLASS.
How to remove the class?
You can use (SETF FIND-CLASS):
(setf (find-class 'foo) nil)
Or, you can use the fancy slime inspector. You call slime-inspect-defintion while pointing the name of the class. Then, you'll see the name. When you select it, you inspect the symbol naming your class. Then, you can see something like:
It names the class FOO [remove]
Provided FOO only names a class, you can use the bigger hammer:
(unintern 'foo)
If I have created instances of the class, is there a way to find them to nullify them and get them GCed?
No, only the GC has the global view, and for practical reasons, it doesn't generally keep backward references about who references a particular object (and how)1.
There is no global record of all instances of a class unless you introduce your own (weak) hash-table to store them. But if you have kept a record of all of your instances, you can CHANGE-CLASS them. For example, you define:
(defclass garbage () ())
... any previously held reference in your object will be released and the GC has an opportunity to dispose of objects your instances where referencing. And when another object reference an instance of 'garbage, you can update it. Instead of using 'garbage, you could probably change instances of old classes to your new class (the name is the same, but the class object is different).
Note also that CHANGE-CLASS is a generic function.
1. Implementations might offer heap walkers. See for example Heap walkers in Allegro CL.

Related

If generic functions work well alone, what is the value of defclass?

I read a lot about generic functions in CL. I get it. And I get why they are valuable.
Mainly, I use them for when I want to execute a similar action with different data types, like this:
(defgeneric build-url (account-key)
(:documentation "Create hunter api urls"))
(defmethod build-url ((key number))
"Build lead api url"
(do-something...))
(defmethod build-url ((key string))
"build campaign api url"
(do-somthing... ))
In this example, campaign-url and lead-url are structures (defstruct).
My question is, at a high level, how do classes add value to the way generic functions + structures work together?
Structures historically predate classes, are more restricted and more "static" than classes: once a structure is defined, the compiler can generate code that accesses its slots efficiently, can assume their layout is fixed, etc. There is a lot of inlining or macro-expansion done that makes it necessary to rebuild everything from scratch when the structure changes. Being able to redefine a struct at runtime is not something defined by the standard, it is merely implementations trying to be nice.
On the other hand, classes have more features and are easier to manipulate at runtime. Suppose you write this class:
(defclass person ()
((name :initarg :name :reader .name)))
And you instantiate it:
(defparameter *someone* (make-instance 'person :name "Julia O'Caml"))
It is possible now to update the class definition:
(defparameter *id-counter* 0)
(defun generate-id ()
(incf *id-counter*))
(defclass person ()
((name :initarg :name :reader .name)
(dob :initarg :date-of-birth :reader .date-of-birth)
(%id :reader .id :initform (generate-id))))
And now, *someone*, which existed already, has two additional fields, dob that is unbound, and %id that is automatically initialized to 1. There is a whole section about Object Creation and Initialization (7.1) that defines how objects can be redefined, change class, etc.
Moreover, this mechanism is not fixed, a lot of the steps described above rely on generic functions. It is possible to define how an object is allocated, initialized, etc. The concept was standardized as what is known as the Meta-Object Protocol, which also introduces the concept of metaobject, the object representing a class: usually a class has a name, parent classes, slots, etc. but you can add new members to a class, or change how instance slots are organized (maybe your just need a global handle and a connection, and the actual instance slots are stored in another process?).
Note also that once CLOS/MOP was defined, it was also eventually possible to define structures in this framework: in the standard , defstruct (without a :type option) defines classes with a structure-class metaclass. Still, they do not behave like standard-class because as said above they are more restricted, and as such are subject to more aggressive compilation optimizations (in general).
Structures are nice if you need to program like in C and you are ok with recompiling all your code when the structure changes. It is however premature optimization to use them in all cases. It is possible to use a lot of standard objects without noticing much slowness nowadays (a bit like Python).

Rcpp-Modules: Understanding .finalizer() and why it doesn't get called

I have written a Rcpp-Module named RMaxima that spawns a child process when it's constructor is called. My motivation for this question is, that I need an object of this class to be destructed, when R exits a functions execution environment in which it has been allocated and bound to a name.
Here are the important parts of my source file:
class RMaxima
{
public:
RMaxima()
{
...
// spawnes child process by instantiating an object
myMaxima = new Maxima(...);
}
~RMaxima()
{
// causes the child process to terminate properly
delete myMaxima;
}
...
private:
Maxima* myMaxima;
};
static void rmaxima_finalizer(RMaxima* ptr)
{
if (ptr)
{
delete ptr;
}
}
RCPP_MODULE(Maxima)
{
class_<RMaxima>("RMaxima")
.constructor()
.method(...)
.finalizer(&rmaxima_finalizer)
;
}
My understanding of garbage collection in R is that when the interpreter exits an environment the value bindings from this environment are thrown away and R's garbage collection frees up memory for any unbound values.
However, this seems to be different for Rcpp-Modules: If I call and exit a function that creates an instance of my class
foo <- function() {
m <- new(Rmaxima)
...
}
then, as expected, m does not appear in the global environment. However, the child process is still running. This means that my class' destructor/ finalizer has not been called. It is called however, when I quit the R session or when I install my corresponding custom package during which loading is tested.
Why? How can I cause it being destructed in a different scope? "Extending R" (Chambers, 2016) gave me some hint
The Rcpp templated type allows conversions to and from "externalptr"
with computations in the C++ code specialized to the type T of the object referred
to. Templated code can in fact use 3 parameters: type, storage scope and a final-
izer to be called when the object pointed to is deleted. The object returned to R
is in all cases of class and type "externalptr". No information is available about
the parametrized form.
as to point out that classes are returned as externalptr and protected from gc(), but I don't see the connection, since object is typeof(m): S4, i.e. a reference class.
Any further hint, where to read about this?
You are using Rcpp Modules which are in a way an alternative to using things yourself with an external pointer. Rcpp offers you Rcpp::XPtr and those are used by a few packages you could look at. (My favourite is a quick GitHub search as in this one across the 'cran' org that mirrors CRAN -- it shows over 1k code hits so some further triage needed.)
And XPtr itself seems to have an open issue in as far as guaranteed access to the finalizer is concerned; I filed issue #1108 on that last year and plan to revisit this 'soon'. But despite that issue which may be a corner case, in general this appears to work. So I would look into XPtr and/or the two helper packages for XPtr use on CRAN.
Lastly, Rcpp Classes were an extension written and contributed by John. They have not seen too much use though. Rcpp Modules is fairly widely used, and there too you could look at examples.
Lastly, you could also do something more simple and direct:
create a class Foo with an empty-ish constructor
have static pointer to a singleton of the class
create an init method to set up the class and have it hold the memory it needs
have setter/getter/worker/result methods as needed
create a teardown method to free the memory
and then call init first (maybe from package load), followed by all the work and then ensure the teardown is called. It's a far-from-perfect design (maybe teardown is not called?) but the simplicity gives you a comparison. And once you have the fragments you can build fancier structures on top.

Kotlin: How are a Delegate's get- and setValue Methods accessed?

I've been wondering how delegated properties ("by"-Keyword) work under-the-hood. I get that by contract the delegate (right side of "by") has to implement a get and setValue(...) method, but how can that be ensured by the compiler and how can those methods be accessed at runtime? My initial thought was that obviously the delegates must me implementing some sort of "SuperDelegate"-Interface, but it appears that is not the case. So the only option left (that I am aware of) would be to use Reflection to access those methods, possibly implemented at a low level inside the language itself. I find that to be somewhat weird, since by my understanding that would be rather inefficient. Also the Reflection API is not even part of the stdlib, which makes it even weirder.
I am assuming that the latter is already (part of) the answer. So let me furthermore ask you the following: Why is there no SuperDelegate-Interface that declare the getter and setter methods that we are forced to use anyway? Wouldn't that be much cleaner?
The following is not essential to the question
The described Interface(s) are even already defined in ReadOnlyProperty and ReadWriteProperty. To decide which one to use could then be made dependable on whether we have a val/var. Or even omit that since calling the setValue Method on val's is being prevented by the compiler and only use the ReadWriteProperty-Interface as the SuperDelegate.
Arguably when requiring a delegate to implement a certain interface the construct would be less flexible. Though that would be assuming that the Class used as a Delegate is possibly unaware of being used as such, which I find to be unlikely given the specific requirements for the necessary methods. And if you still insist, here's a crazy thought: Why not even go as far as to make that class implement the required interface via Extension (I'm aware that's not possible as of now, but heck, why not? Probably there's a good 'why not', please let me know as a side-note).
The delegates convention (getValue + setValue) is implemented at the compiler side and basically none of its resolution logic is executed at runtime: the calls to the corresponding methods of a delegate object are placed directly in the generated bytecode.
Let's take a look at the bytecode generated for a class with a delegated property (you can do that with the bytecode viewing tool built into IntelliJ IDEA):
class C {
val x by lazy { 123 }
}
We can find the following in the generated bytecode:
This is the field of the class C that stores the reference to the delegate object:
// access flags 0x12
private final Lkotlin/Lazy; x$delegate
This is the part of the constructor (<init>) that initialized the delegate field, passing the function to the Lazy constructor:
ALOAD 0
GETSTATIC C$x$2.INSTANCE : LC$x$2;
CHECKCAST kotlin/jvm/functions/Function0
INVOKESTATIC kotlin/LazyKt.lazy (Lkotlin/jvm/functions/Function0;)Lkotlin/Lazy;
PUTFIELD C.x$delegate : Lkotlin/Lazy;
And this is the code of getX():
L0
ALOAD 0
GETFIELD C.x$delegate : Lkotlin/Lazy;
ASTORE 1
ALOAD 0
ASTORE 2
GETSTATIC C.$$delegatedProperties : [Lkotlin/reflect/KProperty;
ICONST_0
AALOAD
ASTORE 3
L1
ALOAD 1
INVOKEINTERFACE kotlin/Lazy.getValue ()Ljava/lang/Object;
L2
CHECKCAST java/lang/Number
INVOKEVIRTUAL java/lang/Number.intValue ()I
IRETURN
You can see the call to the getValue method of Lazy that is placed directly in the bytecode. In fact, the compiler resolves the method with the correct signature for the delegate convention and generates the getter that calls that method.
This convention is not the only one implemented at the compiler side: there are also iterator, compareTo, invoke and the other operators that can be overloaded -- all of them are similar, but the code generation logic for them is simpler than that of delegates.
Note, however, that none of them requires an interface to be implemented: the compareTo operator can be defined for a type not implementing Comparable<T>, and iterator() does not require the type to be an implementation of Iterable<T>, they are anyway resolved at compile-time.
While the interfaces approach could be cleaner than the operators convention, it would allow less flexibility: for example, extension functions could not be used because they cannot be compiled into methods overriding those of an interface.
If you look at the generated Kotlin bytecode, you'll see that a private field is created in the class holding the delegate you're using, and the get and set method for the property just call the corresponding method on that delegate field.
As the class of the delegate is known at compile time, no reflection has to happen, just simple method calls.

Benefits of Virtual Function

The answer
"Calling a non-virtual function will use the function from the same class as the pointer type, regardless of whether the object was actually created as some other derived type. Whereas calling a virtual function will use the function from the original allocated object type, regardless of what kind of pointer you're using."
was the best to me in the question link
What are the differences between overriding virtual functions and hiding non-virtual functions?
However, I still don't see the benefits of making a function virtual. Why not just make it concrete and override the function when necessary?
All the answers in your link are pretty complicated, since they actually answer more questions than actually asked :-)
I try to make it easier (lets hope for the best):
The concept of a virtual function allows you to guarantee that whatever pointer you use (see the example in the link Parent* p2 or Child* cp) on a class with some inheritance involved, it will always call the "youngest" child's implementation in the inheritance chain.
Example: If you have "child -> parent" and "grandchild -> child -> parent" with exact same function f2() definitions and all virtual, you can now assume that "grandchild::f2" is called in all circumstances. If you omitted the "virtual" keyword in your parent, you would have different functions being called, depending on which pointer you use to access the instance.
So. What is this useful for? Imagine you have a template based collection and want to put children inside the collection that is defined as parent-type collection list<Parent*>. If you now call a function on an element you fetch from the list, you can expect the child's function (definition) to be called! If you omit the "virtual" keyword in the f2() definition, the parents function is going to be called, which might be unexpected/undesired in most cases.
Any better? :-)

In smalltalk, deep copy of LinkedList object gives error

This is the smalltalk code I wrote to deep copy two linkedlist object.But when I do this Small talk interpreter raises an error stating that:Unhandled Exception: Message not Understood: nextlink.
list1 add:2.
list2 :=list1 dcopy.
list1 ==list2.
Kindly tell me what is the problem with my code.
This is VisualWorks. LinkedLists are collections used for internal system use and aren't intended for general use. The items added into LinkedLists must subclass from Link (or implement nextLink and nextLink:). You can't add a SmallInteger into a linked list. You can do this:
LinkedList new
add: (LinkValue value: 5);
add: (LinkValue value: 7)
We don't generally use linked lists in Smalltalk. We normally use OrderedCollection instead. If you really need a linked list, add elements which are subclasses of Link.

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