Functional Programming in Swift

Extracting a list of names from a list of Person(s)

Given a Person struct

struct Person {
    let name: String
    let birthYear: Int?
}

and an Array of Person(s)

let persons = [
    Person(name: "Walter White", birthYear: 1959),
    Person(name: "Jesse Pinkman", birthYear: 1984),
    Person(name: "Skyler White", birthYear: 1970),
    Person(name: "Saul Goodman", birthYear: nil)
]

we can retrieve an array of String containing the name property of each Person.

let names = persons.map { $0.name }
// ["Walter White", "Jesse Pinkman", "Skyler White", "Saul Goodman"]

Filtering

Create a stream by selecting the elements from a stream that pass a certain condition is called filtering

var newReleases = [
    [
        "id": 70111470,
        "title": "Die Hard",
        "boxart": "http://cdn-0.nflximg.com/images/2891/DieHard.jpg",
        "uri": "http://api.netflix.com/catalog/titles/movies/70111470",
        "rating": 4.0,
        "bookmark": []
    ],
    [
        "id": 654356453,
        "title": "Bad Boys",
        "boxart": "http://cdn-0.nflximg.com/images/2891/BadBoys.jpg",
        "uri": "http://api.netflix.com/catalog/titles/movies/70111470",
        "rating": 5.0,
        "bookmark": [[ "id": 432534, "time": 65876586 ]]
    ],
    [
        "id": 65432445,
        "title": "The Chamber",
        "boxart": "http://cdn-0.nflximg.com/images/2891/TheChamber.jpg",
        "uri": "http://api.netflix.com/catalog/titles/movies/70111470",
        "rating": 4.0,
        "bookmark": []
    ],
    [
        "id": 675465,
        "title": "Fracture",
        "boxart": "http://cdn-0.nflximg.com/images/2891/Fracture.jpg",
        "uri": "http://api.netflix.com/catalog/titles/movies/70111470",
        "rating": 5.0,
        "bookmark": [[ "id": 432534, "time": 65876586 ]]
    ]
]

var videos1 = [[String: AnyObject]]()
/**
 *  Filtering using map
 */
newReleases.map { e in
    if e["rating"] as! Float == 5.0 {
        videos1.append(["id": e["id"] as! Int, "title": e["title"] as! String])
    }
}

print(videos1)

var videos2 = [[String: AnyObject]]()
/**
 *  Filtering using filter and chaining
 */
newReleases
    .filter{ e in
        e["rating"] as! Float == 5.0
    }
    .map { e in
    videos2.append(["id": e["id"] as! Int, "title": e["title"] as! String])
}

print(videos2)

Projecting

Applying a function to a collection/stream and creating a new collection/stream is called a projection.

/// Projection
var newReleases = [
    [
        "id": 70111470,
        "title": "Die Hard",
        "boxart": "http://cdn-0.nflximg.com/images/2891/DieHard.jpg",
        "uri": "http://api.netflix.com/catalog/titles/movies/70111470",
        "rating": [4.0],
        "bookmark": []
    ],
    [
        "id": 654356453,
        "title": "Bad Boys",
        "boxart": "http://cdn-0.nflximg.com/images/2891/BadBoys.jpg",
        "uri": "http://api.netflix.com/catalog/titles/movies/70111470",
        "rating": [5.0],
        "bookmark": [[ "id": 432534, "time": 65876586 ]]
    ],
    [
        "id": 65432445,
        "title": "The Chamber",
        "boxart": "http://cdn-0.nflximg.com/images/2891/TheChamber.jpg",
        "uri": "http://api.netflix.com/catalog/titles/movies/70111470",
        "rating": [4.0],
        "bookmark": []
    ],
    [
        "id": 675465,
        "title": "Fracture",
        "boxart": "http://cdn-0.nflximg.com/images/2891/Fracture.jpg",
        "uri": "http://api.netflix.com/catalog/titles/movies/70111470",
        "rating": [5.0],
        "bookmark": [[ "id": 432534, "time": 65876586 ]]
    ]
]


var videoAndTitlePairs = [[String: AnyObject]]()
newReleases.map { e in
    videoAndTitlePairs.append(["id": e["id"] as! Int, "title": e["title"] as! String])
}

print(videoAndTitlePairs)

Traversing

let numbers = [3, 1, 4, 1, 5]
// non-functional
for (index, element) in numbers.enumerate() {
    print(index, element)
}

// functional
numbers.enumerate().map { (index, element) in
    print((index, element))
}

Using Filter with Structs

Frequently you may want to filter structures and other complex data types. Searching an array of structs for entries that contain a particular value is a very common task, and easily achieved in Swift using functional programming features. What's more, the code is extremely succinct.

struct Painter {
    enum Type { case Impressionist, Expressionist, Surrealist, Abstract, Pop }
    var firstName: String
    var lastName: String
    var type: Type
}

let painters = [
    Painter(firstName: "Claude", lastName: "Monet", type: .Impressionist),
    Painter(firstName: "Edgar", lastName: "Degas", type: .Impressionist),
    Painter(firstName: "Egon", lastName: "Schiele", type: .Expressionist),
    Painter(firstName: "George", lastName: "Grosz", type: .Expressionist),
    Painter(firstName: "Mark", lastName: "Rothko", type: .Abstract),
    Painter(firstName: "Jackson", lastName: "Pollock", type: .Abstract),
    Painter(firstName: "Pablo", lastName: "Picasso", type: .Surrealist),
    Painter(firstName: "Andy", lastName: "Warhol", type: .Pop)
]

// list the expressionists
dump(painters.filter({$0.type == .Expressionist}))

// count the expressionists
dump(painters.filter({$0.type == .Expressionist}).count)    
// prints "2"

// combine filter and map for more complex operations, for example listing all
// non-impressionist and non-expressionists by surname
dump(painters.filter({$0.type != .Impressionist && $0.type != .Expressionist})
    .map({$0.lastName}).joinWithSeparator(", "))  
// prints "Rothko, Pollock, Picasso, Warhol"