Lambda the Ultimate

In
this paper, we present a typeful approach to implementing objects that makes use of a recently introduced notion of guarded datatypes. In particular, we demonstrate how the feature of multiple inheritance can be supported with this approach, presenting a simple and general account for multiple inheritance in a typeful manner.

[snip]

We refer the reader to [XCC03,Xi02] for further details on this typeful approach to OOP (with single inheritance). The treatment of multiple inheritance in this paper bears a great deal of similarity to the treatment of single inheritance in [XCC03,Xi02], though there are also some substantial differences involved. In order to handle multiple inheritance, we are to treat a path from a (super) class to a (sub)class as a first-class value.

What languages do we wish were still maintained? My list starts:

• Charity
• Cayenne
• O'Haskell
• Kiev
• Cecil

Revisiting coroutines

This paper defends the revival of coroutines as a general control abstraction. After proposing a new classification of coroutines, we introduce the concept of full asymmetric coroutines and provide a precise definition for it through an operational semantics. We then demonstrate that full coroutines have an expressive power equivalent to one-shot continuations and one-shot partial continuations. We also show that full asymmetric coroutines and one-shot partial continuations have many similarities, and therefore present comparable benefits. Nevertheless, coroutines are easier implemented and understood, specially in the realm of procedural languages. Finally, we provide a collection of programming examples that illustrate the use of full asymmetric coroutines to support direct and concise implementation of several useful control behaviors, including cooperative multitasking.

Taking into account real or imaginary interest to control operators on LtU, I believe this paper makes nice reading. See also Coroutines in Lua.

Generics Considered Harmful

Ken Arnold, "programmer and author who helped create Jini, JavaSpaces, Curses, and Rogue", writes that the usefulness of generics is outweighed by their complexity. Ken is talking about Java 5, but such critiques are well-known for C++, and C# is not immune either. Ken describes the Java case as follows:

So, I don't know how to ease into this gently. So I'll just spit it out.

Generics are a mistake.

This is not a problem based on technical disagreements. It's a fundamental language design problem.

Any feature added to any system has to pass a basic test: If it adds complexity, is the benefit worth the cost? The more obscure or minor the benefit, the less complexity its worth. Sometimes this is referred to with the name "complexity budget". A design should have a complexity budget to keep its overall complexity under control.

[...]

Which brings up the problem that I always cite for C++: I call it the "Nth order exception to the exception rule." It sounds like this: "You can do x, except in case y, unless y does z, in which case you can if ..."

Humans can't track this stuff. They are always losing which exception to what other exception applies (or doesn't) in any given case.

[...]

Without [an explicit complexity] budget, it feels like the JSR process ran far ahead, without a step back to ask “Is this feature really necessary”. It seemed to just be understood that it was necessary.

It was understood wrong.

The article contains a few simple supporting examples, including the interesting definition of Java 5's Enum type as:

Enum<T extends Enum<T>>

...which "we're assured by the type theorists ... we should simply not think about too much, for which we are grateful."

If we accept the article's premise, here's a question with an LtU spin: do the more elegant, tractable polymorphic inferencing type systems, as found in functional languages, improve on this situation enough to be a viable alternative that could address these complexity problems? In other words, are these problems a selling point for better type systems, or another barrier to adoption?

[Thanks to Perry Metzger for the pointer.]