Can someone point me to examples of multiparadigm (object-functional) programming in F#?
I am specifically looking for examples which combine OO & functional programming. There's been a lot of talk about how F# is a hybrid language but I've not been able to find examples which demonstrate the example of multiparadigm programming.
Thanks
I made a small (600 lines) Tetris clone with F# that is Object Oriented using XNA. The code is old (uses #light) and isn't the prettiest code you will ever see but it's defiantly a mix of OOP and functional. It consists of ten classes. I don't think I pass any first class functions around but it's a good example of the F#'s functional power in programming the small.
bool stateMenuI made a rough class diagram to help. If you have any questions about it feel free to ask in the comment section.
There are two ways of combining the functional and object-oriented paradigm. To some extent, they are independent and you can write immutable (functional) code that is structured using types (written as F# objects). An F# example of Client type written like this would be:
// Functional 'Client' class
type Client(name, income) =
// Memebers are immutable
member x.Name = name
member x.Income = income
// Returns a new instance
member x.WithIncome(ninc) =
new Client(name, ninc)
member x.Report() =
printfn "%s %d" name income
Note that the WithIncome method (which "changes" the income of the client) doesn't actually do any modifications - it follows the functional style and creates a new, updated, client and returns it as the result.
On the other hand, in F# you can also write object-oriented code that has mutable public properties, but uses some immutable data structure under the cover. This may be useful when you have some nice functional code and want to expose it to C# programmers in a traditional (imperative/object-oriented) way:
type ClientList() =
// The list itself is immutable, but the private
// field of the ClientList type can change
let mutable clients = []
// Imperative object-oriented method
member x.Add(name, income) =
clients <- (new Client(name, income))::clients
// Purely functional - filtering of clients
member x.Filter f =
clients |> List.filter f
(The example is taken from the source code of Chapter 9 of my book. There are some more examples of immutable object-oriented code, for example in parallel simulation in Chapter 14).
The most powerful experience I've had mixing OO (specifically mutation) and functional programming is achieving performance gains by using mutable data-structures internally while enjoying all the benefits of immutability by external users. A great example is an implementation I wrote of an algorithm which yields lexicographical permutations you can find here. The algorithm I use is imperative at it's core (repeated mutation steps of an array) which would suffer if implemented with a functional data structure. By taking an input array, making a read-only copy of it initially so the input is not corrupted, and then yielding read-only copies of it in the sequence expression after the mutation steps of the algorithm are performed, we strike a fine balance between OO and functional techniques. The linked answer references the original C++ implementation as well as benchmarks other purely functional implementation answers. The performance of my mixed OO / functional implementation falls in between the superior performance of the OO C++ solution and the pure functional F# solution.
This strategy of using OO / mutable state internally while keeping pure externally to the caller is used throughout the F# library notably with direct use of IEnumerators in the Seq module.
Another example may be found by comparing memoization using a mutable Dictionary implementation vs. an immutable Map implementation, which Don Syme explores here. The immutable Dictionary implementation is faster but no less pure in usage than the Map implementation.
In conclusion, I think using mutable OO in F# is most powerful for library designers seeking performance gains while keeping everything pure functional for library consumers.
I don't know any F#, but I can show you an example of the exact language mechanics you're looking for in Scala. Ignore it if this isn't helpful.
class Summation {
def sum(aLow : Int, aHigh : Int) = {
(aLow to aHigh).foldLeft(0) { (result, number) => result + number }
}
}
object Sample {
def main(args : Array[String]) {
println(new Summation sum(1, 10))
}
}
I tried to keep it super simple. Notice that we're declaring a class to sum a range, but the implementation is used with a functional style. In this way, we can abstract the paradigm we used to implement a piece of code.
I don't know about F#, but most softwares written in Scala are object-functional in nature.
Scala compiler is probably the largest and a state of the art example of an object-functional software. Other notable examples include Akka, Lift, SBT, Kestrel etc. (Googling will find you a lot more object-functional Scala examples.)
