Haskell

Haskell is a pure functional language with lazy evaluation.

Computational Class

Turing-complete

Programming Paradigms

Defined by

http://www.haskell.org/

Implementations

Things written in Haskell

Commentary

In theory, Haskell is the perfect language for writing reference implementations of programming languages. It, itself, has semantics which are specified reasonably formally. It's purely functional (does not permit side-effects), and these two things bring it much closer to being like mathematics than other languages. Additionally, Haskell programs are lazily evaluated (expressions are only evaluated if they are needed), so, basically, Haskell is denotational semantics. Except it's also a program, so you can run it. It's executable denotational semantics.

Pretty sweet, right? Well... yes, except for the small fact that denotational semantics may not be the best way to describe your language in the first place. You might have to describe I/O and concurrency, for example, and denotational semantics doesn't make that easy.

But I hear you say, well, Haskell has I/O and concurrency features.

OK, look. It's a pretty profound thing to show that you can encode I/O or concurrency or really, any feature of an imperative language that you want, into a lazy functional language using monads, but, that doesn't necessarily mean that it's always a beautiful thing to actually do so.

Yes you can do it, but no, it's not one of Haskell's strengths. I've accepted that.

Using Haskell for other purposes, though? Well, I've already accepted that, when going outside the "batch processing" world, Haskell is a little out of its element, so I would not jump at the opportunity. Armchair category theorists might enjoy writing a multithreaded webserver with shared transactional memory monads or whatever, but I have trouble imagining anyone except a category theorist enjoying maintaining such a beast, so I wouldn't recommend it for most "operational" projects. Choose carefully.

Maybe functional reactive programming, or other techniques, will change this. Or maybe Haskell will simply continue to be the academic playground for type theory research. Or maybe both.