Michael Feathers has a great blog post up today about how object-oriented and functional programming styles are suited to different portions of an application.
Specifically, the object-oriented style fits the high-level application components. The original intention of OO was for objects to pass messages to each other, which is suited to a SOA style and asynchronous message-passing. At this level, components should tell each other what to do, rather than asking for information (as return values). OO and top-down design work well together.
The functional style is better suited to the low-level components. Within objects, a functional style is cleaner and simpler for computations. A functional style is friendlier to an iterative, test-driven approach.
There is one more piece where functional precepts can help us become even more object-oriented, in the message-passing sense. We can make more messages asynchronous when we can pass callbacks. These functions-as-data let us specify what happens next, removing the need to wait for a return value. This looks different from an imperative style, but it is embraced by JavaScript and Node.js, and it’s getting more ubiquitous by the day. It’s suited to asynchronicity, which enables more OO-style message passing.
We can embrace OO and FP in their areas of strength. F# and Scala seem very important in this light. They take away the pain of crossing the seam between OO and FP. We can even embrace the callback (or continuation) style without making our code unreadable: in F#, computation expressions can break imperative-looking code into pieces that execute as each message-pass is completed.
The hybrid languages give us opportunity to optimize our code at both high and low levels, to break our application into large object-oriented chunks and implement each solution functionally. When both paradigms have full language support, we can employ the strengths of each and interoperate at will.