Although I've always wanted to become a professional programmer, I never became one. I studied philosophy and went into a PhD programme in computer science because of my interest in formal logic. I like computers very much, I have professional experience in UNIX administration, and I've done a lot of Perl/Bash/Tcsh scripting, but I've never actually written any non-trivial piece of programming code. Whether you want to model something, verify, or check your proofs, being able to write a computer program that helps you with some task really comes in handy. And then there's this question: what programming language should a person interested in logic/mathematics (without any CS background) learn?
The first obvious choice for people interested in logic is Prolog. It's relatively easy to learn, but after some time playing with it we discover that it's quite limited, and not too efficient. Java is faster, easy as well and more versatile, but for very high-level programming (theorem provers, model checkers) it takes a lot of time to implement ideas, especially when compared to Prolog. Then there are other nice suitable languages like Python (very generic), LISP/Scheme (rather old, yet still very powerful), and finally Haskell.1
Haskell is a general purpose statically- typed purely functional programming language. It supports lazy evaluation, typeclasses and type polymorphism. It provides an enormous set of tools for the programmer, while being far more efficient and flexible than, say, Prolog. Due to its nature it also imposes some discipline on the programmer (for example, static typing prevents a number of errors on the level of compilation), making programming more rigorous and efficient. ESR recently wrote about Haskell:
Haskell is built around a handful of powerful primitive concepts and a pons asinorum. The most pervasive of these, purely functional programming without side effects or variables, is generally described in introductions to Haskell as the most difficult barrier for most programmers arriving from conventional imperative or OO languages like C, C++, and Python. And yes, if your mental model of programming is all about for-loops and static variables — or, for that matter, objects! — learning to think in Haskell is going to be quite a wrench. There are reasons to make the effort. Haskellers (Haskellites? Haskellians? Haskellators? Haskelletons?) maintain that imperative programming relies heavily on side effects that push proving the correctness of programs to somewhere between impractically difficult and impossible. They also like to point out that side effects make programs difficult to automatically parallelize across multiple processors, an increasingly important consideration as multicores become the rule rather than the exception.
That's true, Haskell is not a trivial language to learn. I've been learning it for a while now, and I'm still far from saying that I actually know Haskell. Lazy evaluation and purely functional approach make it especially hard to switch from imperative way of thinking in the beginning. It's difficult to learn, but the effort is worth taking. “Discrete Mathematics Using a Computer” justifies the effort like this:
Pure functional languages like Haskell, as well as mathematics itself, are demanding in that they require you to think through a problem deeply in order to express its solution with equations. It's always possible to hack an imperative program by sticking an assignment in it somewhere in order to patch up a problem, but you cannot do that in a functional program. On the other hand, if our goal is to build correct and reliable software — and this should be our goal! — then the discipline of careful thought will be repaid in higher quality software.2
I can also add that Haskell forces a programmer to study some aspects of theoretical computer science in a way other languages don't. It's a bit like with LISP – you have to know something about lists, trees, recursion, higher- order functions, etc., otherwise you won't be able to learn it. On the other hand I'd risk saying that it's possible to learn for example Perl without getting into theory of programming too much (but perhaps I'm wrong here).
So if it's all so hard, how should one learn Haskell? That very much depends on your level of programming literacy (in general, not necessarily functional programming literacy), amount of time you want to spend, and your motivations behind learning Haskell. I once told my PhD supervisor that I actually know of 3,5 good books on Haskell programming:
“Learn you a Haskell for great good” – not a book really, rather an online tutorial with lots of funny pictures; among the free tutorials it's the best one I know;
“Real world Haskell” – currently the most famous Haskell book I guess; it's a bit like Seibel's “Practical Common Lisp", only on Haskell; even if you're not into web development or databases, this book is the best introduction to Haskell and functional programming I found; oh, and it's free for online reading, but you can always grab a printed copy for ~$50;
“Discrete Mathematics Using a Computer” – this is definitely a must read for anyone interested particularly in using Haskell for solving logical and/or mathematical problems; it's really great and focuses on logic, recursion, set theory and so on, and still provides a good introduction to Haskell;
“The Haskell Road to Logic, Maths and Programming” – another book focusing mainly on logical and mathematical applications; it's good, but I found it very difficult and although authors say they don't assume any background in functional programming, they actually do assume quite a lot (at least that was my impression).
All the books listed above are good, but I think the order in which I put them corresponds to their difficulty level. Not exactly, though: “Real World Haskell” can get pretty dense, but it introduces everything quite gently. “The Haskell Road to Logic…” on the other hand is in my opinion difficult throughout its whole length.
I strongly recommend everyone (especially to fellow logicians) to at least give Haskell a try, it's definitely worth it. And if you happen to live in Bergen and would like to meet to discuss some Haskell-related stuff, there's an informal group of people, consisting of me and three of my friends. Let us know if you'd like to join us in haskelling.