You don't necessarily have to choose between the two paradigms. You can write software with an OO architecture using many functional concepts. FP and OOP are orthogonal in nature.
Take for example C#. You could say it's mostly OOP, but there are many FP concepts and constructs. If you consider Linq, the most important constructs that permit Linq to exist are functional in nature: lambda expressions.
Another example, F#. You could say it's mostly FP, but there are many OOP concepts and constructs available. You can define classes, abstract classes, interfaces, deal with inheritance. You can even use mutability when it makes your code clearer or when it dramatically increases performance.
Many modern languages are multi-paradigm.
As I'm in the same boat (OOP background, learning FP), I'd suggest you some readings I've really appreciated:
Functional Programming for Everyday .NET Development, by Jeremy Miller. A great article (although poorly formatted) showing many techniques and practical, real-world examples of FP on C#.
Real-World Functional Programming, by Tomas Petricek. A great book that deals mainly with FP concepts, trying to explain what they are, when they should be used. There are many examples in both F# and C#. Also, Petricek's blog is a great source of information.
I've been mainly exposed to OO programming so far and am looking forward to learning a functional language. My questions are:
- When do you choose functional programming over object-oriented?
- What are the typical problem definitions where functional programming is a better choice?
If you're in a heavily concurrent environment, then pure functional programming is useful. The lack of mutable state makes concurrency almost trivial. See Erlang.
In a multiparadigm language, you may want to model some things functionally if the existence of mutable state is must an implementation detail, and thus FP is a good model for the problem domain. For example, see list comprehensions in Python or std.range in the D programming language. These are inspired by functional programming.