Simon Peyton Jones

And it becomes almost immutable.

Nobody dares change it anymore.

What they do is it's an immutable piece of software, and you do that stuff around the edges to impedance match what you really want to do to this now immutable blob.

So, of course, you have lots of tests.

So, test your development.

I'm totally with it.

I've done all of that.

But still, GHC, for example, is itself written in ASCII.

It's 35 years old, and yet I do large-scale, systematic refactorings of it, you know, fearlessly, because the type system keeps me safe.

In fact, often what I'll do is I'll change a few types and then start compiling.

And then a sort of wave of changes propagate through forced by, you know, I just get type errors, so I know what to do.

Whereas the thought that I've changed the representation of this data structure a little bit, I've added a field to this data structure, where in the entire compiler might that field be read, written, or freshly allocated?

I can't imagine how anybody maintains 30-year-old software and makes large-scale changes like that without a type system.

It's unimaginable.

So for me, the benefit of type systems is maintainability.

Oh, and designability, right?

So a type, I often write the types of my programs up front.

I write the type, you know, the data types are super perspicuous.

You know, if I say, it's a bit like writing the classes of an arbitrary language, right?

But if you have no types, no classes, nothing, just, I don't know, S expressions, types are my design language.