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 think Haskell has been successful in creating a standard for functional programming languages?

Yes, again not standard as in the ISO standard sense, but standard as a kind of benchmark or brand leader for pure functional languages. It’s definitely been successful in that. If someone asks, ‘tell me the name of a pure functional programming language’, you’d say Haskell. You could say Clean as well, but Clean is less widely used.

How do you respond to criticism of the language, such as this statement from Wikipedia: “While Haskell has many advanced features not found in many other programming languages, some of these features have been criticized for making the language too complex or difficult to understand. In addition, there are complaints stemming from the purity of Haskell and its theoretical roots.”

Partly it’s a matter of taste. Things that one person may find difficult to understand, another might not. But also it’s to do with doing one thing well again. Haskell does take kind of an extreme approach: the language is pure, and it has a pretty sophisticated type system too. We’ve used Haskell in effect as a laboratory for exploring advanced type system ideas. And that can make things complicated.

I think a good point is that Haskell is a laboratory: it’s a lab to explore ideas in. We intended it to be usable for real applications, and I think that it definitely is, and increasingly so. But it wasn’t intended as a product for a company that wanted to sell a language to absolutely as many programmers as possible, in which you might take more care to make the syntax look like C, and you might think again about introducing complex features as you don’t want to be confusing.

Haskell was definitely designed with programmers in mind, but it wasn’t designed for your average C++ programmer. It’s to do not with smartness but with familiarity; there’s a big mental rewiring process that happens when you switch from C++ or Perl to Haskell. And that comes just from being a purely functional language, not because it’s particularly complex. Any purely functional language requires you to make that switch.

If you’re to be a purely functional programming language, you have to put up with that pain. Whether it’s going to be the way of the future and everybody will do it – I don’t know. But I think it’s worth some of us exploring that. I feel quite unapologetic about saying that’s what Haskell is – if you don’t want to learn purely functional programming or it doesn’t feel comfortable to you or you don’t want to go through the pain of learning it, well, that’s a choice you can make. But it’s worth being clear about what you’re doing and trying to do it in a very clear and consistent and continuous way.

Haskell, at least with GHC, has become very complicated. The language has evolved to become increasingly complicated as people suggest features, and we add them, and they have to interact with other features. At some point, maybe it will become just too complicated for any mortal to keep their head around, and then perhaps it’s time for a new language – that’s the way that languages evolve.

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