I'm a very experienced Object Oriented developer. Which of the Functional Programming languages would be the best one for getting my feet wet? Keeping in mind:
Which Functional Programming language would you recommend?
Scala?
Good IDE support, Java class library and debugging tools. And it's OO/functional, not just functional. And it runs in the JVM.
I would take a very long look at F# for this. It's got Visual Studio as the IDE which is fantastic and a pile of resources.
A few of our guys have started using it and exploring the power of the language.
I would consider F# - you get the best of both worlds.
http://research.microsoft.com/en-us/um/cambridge/projects/fsharp/
Javascript is very good for learning and using functional programming.
You implicitly use it when setting an onclick for an html element.
It's close enough to the C style syntax to not have a lot of extra confsion because of syntax.
It can be used in the browser and on the command line through Java.
For a good overview, watch the Crockford videos from YUI theater.
If you're familiar with Visual Studio and .NET, then F# would be a great bet: http://research.microsoft.com/en-us/um/cambridge/projects/fsharp/about.aspx. Of your three criteria, it's got the IDE and debugging tools, but not compiler maturity. I'd look to Lisp (Common Lisp or Scheme) for that.
Seems to me that if you're an experienced OO programmer, you should learn a functional programming language that does not let you keep on programming in the OO style.
My recommendation is Haskell (although some OO is possible in it too, it's not the typical idiom.) Haskell has mature compilers (ghc) and good testing tools (Quickcheck, etc).
Even if i'm going to be down-voted, i've to say O'caml...
IDE: emacs Compiler: pretty good Debugging tools: not so much
but it has a kind of object-oriented flavor that would be good for the transition ....
(otherwise go for F#)
I forgot, there's Nemerle also..a mix of C# with functional programming (I don't what is the status of this project though..)
I'd recommend F# (When you're under windows). It has a good IDE (Visual Studio) and debugger, can use the complete .NET-Framework and isn't purely functional like Haskell, but will help getting into functional programming by encompassing object-oriented and some imperative structures. And since it's still being developed and promoted by Microsoft, it won't be soo isolated as Haskell. I think, it has got the biggest "real-world-applicability" of all functional programming languages.
The Visual Studio support for F# is a very significant factor. Because F# is strongly typed intellisense picks up errors immediately, and the tool tips showing function types (eg. "int -> bool") are invaluable. Getting to grips with the functional type system is probably the biggest barrier you will face, so, if you've got Visual Studio, then this should be a major plus for F#. (Intellisense just for learning the syntax is also very helpful)
On the other hand, whenever I look for on-line resources on a functional topic (eg. pattern matching, curry functions), I am impressed by the depth of the OCaml community. When I got started with this a year ago, the on-line tutorials and manuals for OCaml were vastly superior to F# (and probably still are).
I actually got to grips with F# by working OCaml tutorials in Visual Studio :)
[Edit, May 2010] With the changes to F# in the last year, I would no longer recommend OCaml for learning F#. The differences are just too significant, and would require you to "unlearn" a lot of OCaml. Best to go straight for one of the F# resources.
I've worked with both Scheme and some SML-NJ (Standard ML / New Jersey) in the past.
But I would highly recommend picking up The Little Schemer or The Little MLer by Friedman and Felleisen. They start really basic and work up to lambdas and such, in a picture book kind of way.
Given the popularity of OCaml and F# (both in the ML family), The Little MLer might be more what you want.
If you go with Scheme though (and I love Scheme), the SICP is also a great book. I still have mine from college. Friedman, Felleisen, et. al. also have two other Scheme books in their series.
I suggest giving Lisp/scheme a try. Just because it's so different from anything else. Practical Common Lisp is a wonderful introduction.
I'd check into LISP. It allows you to flexibly combine what you know of OO with what you want to learn of Functional Programming styles. As Sander said, Practical Common Lisp is a great start, and the Lisp Hyperspec documents well all of the myriad of Common Lisp functions.