How to write Extension Methods in F# and Swift

I’ve used extension methods for a while in F# as a neat way of adding utility methods to system types. For example, suppose you have a set and want to add some elements to it? You might write something like this:

  let collection = ... // initial set defined elsewhere
  let collection = // add extra elements into collection
    [| 1; 2; 3 |] |> Array.fold (fun (acc : Set<'T>) item -> acc.Add item) collection

You might find yourself writing this snippet frequently and for different container types. Instead of defining the operation every time, it’s better to write an extension method:

namespace MusingStudio.Extensions
module Set =
  let AddMulti (items : seq<'T>) (collection : Set<'T>) =
    items |> Seq.fold (fun (acc : Set<'T>) item -> acc.Add item) collection

Then, by bringing the extensions into scope, we can call AddMulti as if it were part of the system Set interface:

open MusingStudio.Extensions

[<EntryPoint>]
let main argv = 
    let initial = [| 1; 2; 3 |] |> Set.ofArray
    let updated = initial |> Set.AddMulti [| 4; 5; 6 |]
    printfn "%A" updated
    0 // return an integer exit code

I was looking for a way to get the current time/date in Swift, and found some code on Stack Overflow that uses Extension Methods to do it. I’ve added a couple more methods to get the seconds and day-of-month:

import Foundation

// From: http://stackoverflow.com/questions/24070450/how-to-get-the-current-time-and-hour-as-datetime
extension NSDate
{
    func hours() -> Int
    {
        //Get Hours
        let calendar = NSCalendar.currentCalendar()
        let components = calendar.components(.Hour, fromDate: self)
        let hours = components.hour
        
        //Return Hour
        return hours
    }
    
    func minutes() -> Int
    {
        //Get Minutes
        let calendar = NSCalendar.currentCalendar()
        let components = calendar.components(.Minute, fromDate: self)
        let minutes = components.minute
        
        //Return Minute
        return minutes
    }
    
    func seconds() -> Int
    {
        // Get Seconds
        let calendar = NSCalendar.currentCalendar()
        let components = calendar.components(.Second, fromDate: self)
        let seconds = components.second
        
        return seconds
    }
    
    func day() -> Int
    {
        // Get Day
        let calendar = NSCalendar.currentCalendar()
        let components = calendar.components(.Day, fromDate: self)
        let day = components.day
        
        return day
    }
}

This simplifies the interface to NSDate:

  // Get the time properties
  let currentDate = NSDate()
  let minutes = currentDate.minutes()
  let hours = currentDate.hours()
  let seconds = currentDate.seconds()
  let dayOfMonth = currentDate.day()