The A-Z of Programming Languages: Haskell

Simon Peyton-Jones tells us why he is most proud of Haskell's purity, type system and monads.

Simon Peyton-Jones

Simon Peyton-Jones

Do you ever think the joke about Microsoft using Haskell as its standard language had come true? Haskell.NET?

Well, there are two answers to this one – the first would be of course, yes, that would be fantastic! I really think that functional programming has such a lot to offer the world.

As for the second, I don’t know if you know this, but Haskell has a sort of unofficial slogan: avoid success at all costs. I think I mentioned this at a talk I gave about Haskell a few years back and it’s become sort of a little saying. When you become too well known, or too widely used and too successful (and certainly being adopted by Microsoft means such a thing), suddenly you can’t change anything anymore. You get caught and spend ages talking about things that have nothing to do with the research side of things.

I’m primarily a programming language researcher, so the fact that Haskell has up to now been used for just university types has been ideal. Now it’s used a lot in industry but typically by people who are generally flexible, and they are a generally a self selected rather bright group. What that means is that we could change the language and they wouldn’t complain. Now, however, they’re starting to complain if their libraries don’t work, which means that we’re beginning to get caught in the trap of being too successful.

Haskell has a sort of unofficial slogan: avoid success at all costs

What I’m really trying to say is that the fact Haskell hasn’t become a real mainstream programming language, used by millions of developers, has allowed us to become much more nimble, and from a research point of view, that’s great. We have lots of users so we get lots of experience from them. What you want is to have a lot of users but not too many from a research point of view – hence the avoid success at all costs.

Now, but at the other extreme, it would be fantastic to be really, really successful and adopted by Microsoft. In fact you may know my colleague down the corridor, Don Syme, who designed a language: F#. F# is somewhere between Haskell and C# - it’s a Microsoft language, it’s clearly functional but it’s not pure and it’s defining goal is to be a .NET language. It therefore takes on lots of benefits and also design choices that cannot be changed from .NET. I think that’s a fantastic design point to be in and I’m absolutely delirious that Don’s done that, and that he’s been successfully turning it into a product – in some ways because it takes the heat off me, as now there is a functional language that is a Microsoft product!

So I’m free to research and do the moderate success thing. When you talk to Don [in a forthcoming interview in the A-Z of Programming Languages series], I think you will hear him say that he’s got a lot of inspiration from Haskell. Some ideas have come from Haskell into F#, and ideas can migrate much more easily than concrete syntax and implementation and nitty gritty design choices.

Haskell is used a lot for educational purposes. Are you happy with this, being a former lecturer, and why do you think this is?

Functional programming teaches you a different perspective on the whole enterprise of writing programs. I want every undergraduate to learn to write functional programs. Now if you’re going to do that, you have to choose if you are going to teach Haskell or ML or Clean. My preference would be Haskell, but I think the most important thing is that you should teach purely functional programming in some shape or form as it makes you a better imperative programmer. Even if you’re going to go back to C++, you’ll write better C++ if you become familiar with functional programming.

Have you personally taught Haskell to many students?

No, I haven’t actually! While I was at Glasgow I was exclusively engaged in first year teaching of Ada, because that was at the time in the first year language that Glasgow taught, and Glasgow took the attitude that each senior professor should teach first year students, as they’re the ones that need to be turned on and treated best. That’s the moment when you have the best chance of influencing them – are they even gong to take a second year course?

Did you enjoy teaching Ada?

Yes, it was a lot of fun. It’s all computer science and talking to 200 undergraduates about why computing is such fun is always exciting.

You’ve already touched on why you think all programmers should learn to write functional programs. Do you think functional programming should be taught at some stage in a programmer’s career, or it should be the first thing they learn?

I don’t know – I don’t actually have a very strong opinion on that. I think there are a lot of related factors, such as what the students will put up with! I think student motivation is very important, so teaching students a language they have heard of as their first language has a powerful motivational factor.

On the other hand, since students come with such diverse experiences (some of them have done heaps of programming and some of them have done none) teaching them a language which all of them aren’t familiar with can be a great leveler. So if I was in a university now I’d be arguing the case for teaching functional programming as a first year language, but I don’t think it’s a sort of unequivocal, “only an idiot would think anything else” kind of thing!

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