Elmord's Magic Valley

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Object model and dot syntax in Hel

2019-04-01 20:43 -0300. Tags: comp, prog, pldesign, hel, fenius, in-english

[Despite the date, this is not an April Fool's joke. This is mostly a mind dump for future reference.]

I have written about noun-centric vs. verb-centric OO before (in Portuguese), but the question surfaces now in the context of Hel's design.

Most mainstream OO programming languages are noun-centric: methods (verbs) belong to objects (nouns). When calling x.foo(y), the method to be called is determined by the (dynamic) class of x; the call can be conceptualized as sending the message foo(y) to the object x.

By contrast, Common Lisp and other languages influenced by the Common Lisp Object System (CLOS) are verb-centric: methods (verbs) are entities in their own right, which can be applied to objects (nouns). Methods of the same name are grouped under a generic function. The method calling syntax is typically the same as the regular function call syntax: (foo x y). The method invoked by a call to a generic function is determined by the (dynamic) classes of all arguments, not just x. New methods can be defined at any time, since they are independent from the class. The class definition, on the other hand, contains just the fields and the superclasses (and metaclass, and those sorts of thing), but no methods.

In Dylan, x.foo(y) is syntactic sugar for foo(x, y). This way you can have both the familiar method call notation and the verb-centric nature of CLOS.

Now, everything in language design is tradeoffs, and here we have some.

Namespacing

One of the main differences between the noun-centric and verb-centric models is in how they define namespaces for methods.

Suppose we define a File class in a module, with a method size() returning the file's size in bytes. In another module, we define a Circle class, with a method size() returning the circle's size in pixels. (Okay, we could have called the circle method radius or diameter(), but let's suppose the module was written by someone else and we don't control the name.)

In noun-centric OO, the class creates a namespace for its methods. someFile.size() and someCircle.size() are entirely different methods, because someFile and someCircle belong to different classes. By contrast, in verb-centric OO, these calls would be syntactic sugar for size(someFile) and size(someCircle); this would only work if there was a single generic function size encompassing both methods, which does not make much sense in this example (since size means something completely different in each class).

Common Lisp solves this problem by having the names belong to packages: variable names are symbols, and each symbol belongs to a package. In this case, each module/package would have its own symbol size, and there would be two distinct generic functions, both named size, but each by a distinct size. Due to the way the package system works in Common Lisp, you would not be able to import both at the same time: you would have to use a fully qualified symbol name to refer to at least one of them.

Guile does something different: if you import two generic functions with the same name into a module, they are merged into a new generic function combining the methods of both. In this case, even though each module defines its own generic function size, a module importing both would see a single generic function size which would accept both files and circles. May seem a bit weird from a conceptual standpoint, but it works nicely. Without Guile's trickery, the Schemely solution would be to rename one (or both) of the functions when importing (the equivalent of Python's from file import size as file_size). I don't know how Dylan handles this situation.

The flip side is that noun-centric OO provides a single namespace for all of a class' methods. This means that you have to be careful about overriding methods in subclasses. Suppose someone defines a class A and I create a subclass B inheriting from A and define a method foo on it. In the future, the author of class A decides to add a method foo to A. Now my class B inadvertently overrides the foo method of the superclass, just because it happens to have the same name as A's new foo method. Some noun-centric OO languages like C# require the explicit use of an override keyword on overriding methods to avoid this kind of accidental override. By contrast, in the CLOS world, my definition of the generic function foo would be unrelated to the new foo created by class A's author, so no conflict would ensue. (A package import conflict might happen, though. And if all of the symbols in the package where A is defined are imported into the package where B is defined, you might end up using the same symbol for both foos without even realizing. Yeah, packages are fun like that. But at least it's possible to have two different, non-conflicting foo methods.)

Another way in which noun-centric OO provides a namespace for methods is by separating method names from regular variables. This means I can write let size = file.size() without losing access to the size method. In Common Lisp this problem does not arise because functions/methods live in a different namespace from regular variables anyway, but I'm not willing to go that route. In Scheme, the local size would shadow the global method. (Again, I don't know how Dylan handles this.)

Yet another consequence of the namespacing thing is that, in the noun-centric model, you don't have to import a class' methods individually: if you have access to the class, you have access to all of its (public) methods. In the verb-centric model, generic functions are independent entities, and would have to be imported individually (or else you import all of them at once by importing the whole module (the equivalent of Python's from foo import *), thus polluting your module's namespace).

A possible counter-argument against the noun-centric model is that importing all of a class' methods is kind of an illusion: there are typically functions taking objects of a given class as arguments which are not methods of the class, and those would have to be imported manually anyway. In practice, though, the most common operations on a given object will be methods of the object, so this argument may not be very strong.

The last point brings an advantage of the verb-centric model: you can 'add' methods to a class without modifying its source, since the methods are independent entities that can be defined anywhere, just like regular functions. Some languages, such as Ruby, have "open classes" to which methods can be added at any time. One problem with this is that no matter where the method definitions are for a given class, they all share the same method namespace, so conflicts may happen more often. The other problem is that the set of methods available in a class depends on which modules have been loaded. This is also the case in the verb-centric model, but at least it's completely explicit: you only have access to a method if you import it. In the Ruby model, you see every method in a class regardless of where it was defined, which may create implicit module dependencies (i.e., I use a method defined elsewhere, but I don't import the defining module explicitly, it just happens to be available by the time my code runs).

If I understand correctly, Haskell's typeclasses offer an alternative model: you can instantiate a typeclass (i.e., implement an interface) anywhere, and even implement the same interface multiple times in different ways, but you only see the implementations if you import the implementing module. Transplanting this model to class definition, you might be able to add methods to a class anywhere, but would only see the new methods if you import the defining module. I'm not sure this would work; it seems plausible in a static world, but not really when you can obtain an object from anywhere and call a method on it without knowing its type (or worse, via reflection).

Conclusion

I intend to implement a rudimentary object model for Hel soon. I'm leaning towards plain old noun-centric OO, if only because it's easier to reach a class' methods (you don't have to import each method individually), and because it limits conflicts between local variables and method names. Let's see how it goes.

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