Skip to main content

Visitor Pattern Deprecated: First-Class Messages Are All You Need!

In previous posts [1,2], I argued that the visitor pattern was a verbose, object-oriented equivalent of a pattern matching function. The verbosity stems from the need to add the dispatching code to each data class in the hierarchy, and because the encapsulation inherent to objects is awkward when dealing with pure data.

A single addition to an OOP language could completely do away with the need for the dispatching code and make OO pattern matching simple and concise: first-class messages (FCM). By this I mean, messages sent to an object are themselves 'objects' that can be passed around as parameters.

To recap, functional languages reify the structure of data (algebraic data types), and they abstract operations (functions). OOP languages reify operations (interfaces), but they abstract the structure of data (encapsulation). They are duals of one another.

All the Exp data classes created in [1,2] shouldn't be classes at all, they should be messages that are sent to the IVisitor object implementing the pattern matching:

data Exp = App(e,e) | Int(i) | ...

IVisitor iv = ...

//the expression: 1 + 2
Exp e = App(Plus(Int(1), Int(2)));

//'e' is now the "App" operation placed in a variable
iv.e //send "App" to visitor

FCM removes the inefficient double-dispatch inherent to the visitor pattern, while retaining object encapsulation for when you really need it; the best of both worlds.

Note also how the above message declaration looks a great deal like a variant in OCaml? This is because first-class messages are variants, and pattern-matching functions are objects. I'm actually implementing the DV language in that paper, first as an interpreter, then hopefully as a compiler for .NET. To be actually useful as a real language, DV will require some extensions though, so stay tuned. :-)

[Edit: made some clarifications to avoid confusion]


Popular posts from this blog

async.h - asynchronous, stackless subroutines in C

The async/await idiom is becoming increasingly popular. The first widely used language to include it was C#, and it has now spread into JavaScript and Rust. Now C/C++ programmers don't have to feel left out, because async.h is a header-only library that brings async/await to C! Features: It's 100% portable C. It requires very little state (2 bytes). It's not dependent on an OS. It's a bit simpler to understand than protothreads because the async state is caller-saved rather than callee-saved. #include "async.h" struct async pt; struct timer timer; async example(struct async *pt) { async_begin(pt); while(1) { if(initiate_io()) { timer_start(&timer); await(io_completed() || timer_expired(&timer)); read_data(); } } async_end; } This library is basically a modified version of the idioms found in the Protothreads library by Adam Dunkels, so it's not truly ground bre

Building a Query DSL in C#

I recently built a REST API prototype where one of the endpoints accepted a string representing a filter to apply to a set of results. For instance, for entities with named properties "Foo" and "Bar", a string like "(Foo = 'some string') or (Bar > 99)" would filter out the results where either Bar is less than or equal to 99, or Foo is not "some string". This would translate pretty straightforwardly into a SQL query, but as a masochist I was set on using Google Datastore as the backend, which unfortunately has a limited filtering API : It does not support disjunctions, ie. "OR" clauses. It does not support filtering using inequalities on more than one property. It does not support a not-equal operation. So in this post, I will describe the design which achieves the following goals: A backend-agnostic querying API supporting arbitrary clauses, conjunctions ("AND"), and disjunctions ("OR"). Implemen

Simple, Extensible IoC in C#

I just committed the core of a simple dependency injection container to a standalone assembly, Sasa.IoC . The interface is pretty straightforward: public static class Dependency { // static, type-indexed operations public static T Resolve<T>(); public static void Register<T>(Func<T> create) public static void Register<TInterface, TRegistrant>() where TRegistrant : TInterface, new() // dynamic, runtime type operations public static object Resolve(Type registrant); public static void Register(Type publicInterface, Type registrant, params Type[] dependencies) } If you were ever curious about IoC, the Dependency class is only about 100 lines of code. You can even skip the dynamic operations and it's only ~50 lines of code. The dynamic operations then just use reflection to invoke the typed operations. Dependency uses static generic fields, so resolution is pretty much just a field access + invoking a