The Universe of Discourse

Atypical Typing
I just got back from Nashville, Tennessee, where I delivered a talk at OOPSLA 2008, my first talk as an "invited speaker". This post is a bunch of highly miscellaneous notes about the talk.

If you want to skip the notes and just read the talk, here it is.

Talk abstract

Many of the shortcomings of Java's type system were addressed by the addition of generics to Java 5.0. Java's generic types are a direct outgrowth of research into strong type systems in languages like SML and Haskell. But the powerful, expressive type systems of research languages like Haskell are capable of feats that exceed the dreams of programmers familiar only with mainstream languages.

In this talk I'll give a brief retrospective on the history of type systems and an introduction to the type system of the Haskell language, including a remarkable example where the Haskell type checker diagnoses an infinite loop bug at compile time.

In 1999, the claim that strong typing does not have to suck was surprising news, and particularly so to Perl Mongers. In 2008, however, this argument has been settled by Java 5, whose type system demonstrates pretty conclusively that strong typing doesn't have to suck. I am not saying that you must like it, and I am not saying that there is no room for improvement. Indeed, it's obvious that the Java 5 type system has room for improvement: if you take the SML type system of 15 years ago, and whack on it with a hammer until it's chipped and dinged all over, you get the Java 5 type system; the SML type system of the early 1990s is ipso facto an improvement. But that type system didn't suck, and neither does Java's.

So I took out the arguments about how static typing didn't have to suck, figuring that most of the OOPSLA audience was already sold on this point, and took a rather different theme: "Look, this ivory-tower geekery turned out to be important and useful, and its current incarnation may turn out to be important and useful in the same way and for the same reasons."

In 1999, I talked about Hindley-Milner type systems, and although it was far from clear at the time that mainstream languages would follow the path blazed by the HM languages, that was exactly what happened. So the HM languages, and Haskell in particular, contained some features of interest, and, had you known then how things would turn out, would have been worth looking at. But Haskell has continued to evolve, and perhaps it still is worth looking at.

Or maybe another way to put it: If the adoption of functional programming ideas into the mainstream took you by surprise, fair enough, because sometimes these things work out and sometimes they don't, and sometimes they get adopted and sometimes they don't. But if it happens again and takes you by surprise again, you're going to look like a dumbass. So start paying attention!

Haskell types are hard to explain

It is a lot easier to explain ML's type system than it is to explain Haskell's. Partly it's because ML is simpler to begin with, but also it's because Haskell is so general and powerful that there are very few simple examples! For example, in SML one can demonstrate:

        (* SML *)
        val 3 : int;
        val 3.5 : real;

But in Haskell, 3 has the type (Num t) ⇒ t, and 3.5 has the type (Fractional t) ⇒ t. So you can't explain the types of literal numeric constants without first getting into type classes.

The benefit of this, of course, is that you can write 3 + 3.5 in Haskell, and it does the right thing, whereas in ML you get a type error. But it sure does make it a devil to explain.

Similarly, in SML you can demonstrate some simple monomorphic functions:

	  not : bool → bool
	 real : int → real
	 sqrt : real → real
	floor : real → int

            not :: Bool → Bool
    fromInteger :: (Num a) ⇒ Integer → a    -- analogous to 'real'
           sqrt :: (Floating a) ⇒ a → a
          floor :: (RealFrac a, Integral b) ⇒ a → b

Slides

Conference plenary sessions

On the other hand...

McCarthy didn't actually make it, unfortunately. But I did get to meet Richard Gabriel and Gregor Kiczales. And Daniel Weinreb, although I didn't know who he was before I met him. But now I'm glad I met Daniel Weinreb. During my talk I digressed to say that anyone who is appalled by Perl's regular expression syntax should take a look at Common Lisp's format feature, which is even more appalling, in much the same way. And Weinreb, who had been sitting in the front row and taking copious notes, announced "I wrote format!".

More explaining of jokes

One joke, however, is too amusing to leave out. At the start of the talk, I pretended to have forgotten my slides. "No problem," I said. "All my talks these days are generated automatically by the computer anyway. I'll just rebuild it from scratch." I then displayed this form, which initialliy looked like this:

Wadler's anecdote

I had never been to OOPSLA, so I also asked Wadler what the OOPSLA people would want to hear about. He mentioned STM, but since I don't know anything about STM I didn't say anything about it.

View it online

[ Addendum 20081031: Thanks to a Max Rabkin for pointing out that Haskell's analogue of real is fromInteger. I don't know why this didn't occur to me, since I mentioned it in the talk. Oh well. ]

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