Monday, October 1, 2007

Standard Object

I'm a fan of CLOS. When I wrote the .NET interface to Larceny, I based it on Eli Barzilay's Swindle, which is based on Tiny CLOS. Naturally, if I'm writing scripting code in Lisp, I want to have the same sort of interface to the underlying object layer.

CLOS has a couple of interesting requirements that can be tricky to implement. The first is the ability to construct funcallable-standard-instances, that is, first-class procedure objects that can have additional data associated with them. The second is the ability to call change-class on an instance to transform it in place to an instance of another class. (Why would I want to do that?! Normally, a user of CLOS wouldn't want to or need to do something so horrendous to an object, but if you are using a REPL and change a class definition by reloading a file, you want the system to update existing instances to the new schema. Rather than forcing the user to exit the REPL, recompile, reload, and then reconstruct his state, we tweak the objects behind the scenes and keep going.) If we combine these features we can potentially turn any instance into a procedure. (No, we don't want to design features that deliberately use this ability, we want to allow the user to type defclass..., say `Oops! I meant defgeneric...,' and do the fixup on the fly.)

The CLR has first-class functions called delegates. A delegate is basically an object that contains an object and a reference to a method implemented by the object. When the delegate is invoked, the method within the object is invoked and the answer is returned. If we consider the method to be a piece of code and the object to be an environment, then we can see that a CLR delegate is simply a closure. If we use delegates to implement funcallable-standard-instances, then our CLOS generic functions can be used anywhere a delegate can (modulo type co- and contra- variance). A non-funcallable standard-instance can simply be a delegate that refers to a method that raises an exception.

We need to associate some additional data with our standard-instances. First, an instance has an associated class object. While it is true that our delegates have an associated .NET type, it is always going to be the System.Delegate type and the .NET types are insufficient for our purposes. Second, an instance has an associated set of `slots'. We need the ability to both refer to these elements and to replace them in order to implement change-class. There are a couple of strategies that will work here. One is to hold the additional data in a weak hash table keyed by the delegate. This technique was used in the original version of Swindle, but the hash lookup will incur a fair amount of overhead. A portable technique is to define a helper class with the necessary fields:

class ManifestInstance
{
    StandardObject class;
    object [] slots;
    function onFuncall;
}


A StandardObject would be a delegate to a trampoline function in ManifestInstance. This technique works, but it seems kind of a kludge. We'd like to have our own special subclass of delegate that simply has two additional fields. (We'll have an additional benefit: the debugger will show us the class and slots as immediate subvalues in the delegate. If we indirect through a ManifestInstance, we have to manually chase down the pointer in the debugger.) We can create subclasses of delegates by using the delegate declaration in C#, but we can't add any new fields.

We can, however, do this trick in IL and assemble it into a signed assembly that can be placed in the GAC. We can then just add a reference to it in VC#.

I've added some code for this in http://jrm-code-project.googlecode.com/svn/trunk/Jrm.StandardObject/. I'm still tweaking it a bit, but it seems to be working.
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