arrays - with - type 'element' constrained to non-protocol, non-class type




How can I extend typed Arrays in Swift? (6)

(Swift 2.x)

You can also extend the array to conform to a protocol containing blue-rpints for generic type methods, e.g., a protocol containing your custom functional utils for all generic array elements conforming to some type constraint, say protocol MyTypes. The bonus using this approach is that you can write functions taking generic array arguments, with a constraint that these array arguments must conform to your custom function utilities protocol, say protocol MyFunctionalUtils.

You can get this behaviour either implicitly, by type constraining the array elements to MyTypes, or---as I will show in the method I describe below---, quite neatly, explicitly, letting your generic array functions header directly show that input arrays conforms to MyFunctionalUtils.


We begin with Protocols MyTypes for use as type constraint; extend the types you want to fit in your generics by this protocol (example below extends fundamental types Int and Double as well as a custom type MyCustomType)

/* Used as type constraint for Generator.Element */
protocol MyTypes {
    var intValue: Int { get }
    init(_ value: Int)
    func *(lhs: Self, rhs: Self) -> Self
    func +=(inout lhs: Self, rhs: Self)
}

extension Int : MyTypes { var intValue: Int { return self } }
extension Double : MyTypes { var intValue: Int { return Int(self) } }
    // ...

/* Custom type conforming to MyTypes type constraint */
struct MyCustomType : MyTypes {
    var myInt : Int? = 0
    var intValue: Int {
        return myInt ?? 0
    }

    init(_ value: Int) {
        myInt = value
    }
}

func *(lhs: MyCustomType, rhs: MyCustomType) -> MyCustomType {
    return MyCustomType(lhs.intValue * rhs.intValue)
}

func +=(inout lhs: MyCustomType, rhs: MyCustomType) {
    lhs.myInt = (lhs.myInt ?? 0) + (rhs.myInt ?? 0)
}

Protocol MyFunctionalUtils (holding blueprints our additional generic array functions utilities) and thereafter, the extension of Array by MyFunctionalUtils; implementation of blue-printed method(s):

/* Protocol holding our function utilities, to be used as extension 
   o Array: blueprints for utility methods where Generator.Element 
   is constrained to MyTypes */
protocol MyFunctionalUtils {
    func foo<T: MyTypes>(a: [T]) -> Int?
        // ...
}

/* Extend array by protocol MyFunctionalUtils and implement blue-prints 
   therein for conformance */
extension Array : MyFunctionalUtils {
    func foo<T: MyTypes>(a: [T]) -> Int? {
        /* [T] is Self? proceed, otherwise return nil */
        if let b = self.first {
            if b is T && self.count == a.count {
                var myMultSum: T = T(0)

                for (i, sElem) in self.enumerate() {
                    myMultSum += (sElem as! T) * a[i]
                }
                return myMultSum.intValue
            }
        }
        return nil
    }
}

Finally, tests and two examples showing a function taking generic arrays, with the following cases, respectively

  1. Showing implicit assertion that the array parameters conform to protocol 'MyFunctionalUtils', via type constraining the arrays elements to 'MyTypes' (function bar1).

  2. Showing explicitly that the array parameters conform to protocol 'MyFunctionalUtils' (function bar2).

The test and examples follows:

/* Tests & examples */
let arr1d : [Double] = [1.0, 2.0, 3.0]
let arr2d : [Double] = [-3.0, -2.0, 1.0]

let arr1my : [MyCustomType] = [MyCustomType(1), MyCustomType(2), MyCustomType(3)]
let arr2my : [MyCustomType] = [MyCustomType(-3), MyCustomType(-2), MyCustomType(1)]

    /* constrain array elements to MyTypes, hence _implicitly_ constraining
       array parameters to protocol MyFunctionalUtils. However, this
       conformance is not apparent just by looking at the function signature... */
func bar1<U: MyTypes> (arr1: [U], _ arr2: [U]) -> Int? {
    return arr1.foo(arr2)
}
let myInt1d = bar1(arr1d, arr2d) // -4, OK
let myInt1my = bar1(arr1my, arr2my) // -4, OK

    /* constrain the array itself to protocol MyFunctionalUtils; here, we
       see directly in the function signature that conformance to
       MyFunctionalUtils is given for valid array parameters */
func bar2<T: MyTypes, U: protocol<MyFunctionalUtils, _ArrayType> where U.Generator.Element == T> (arr1: U, _ arr2: U) -> Int? {

    // OK, type U behaves as array type with elements T (=MyTypes)
    var a = arr1
    var b = arr2
    a.append(T(2)) // add 2*7 to multsum
    b.append(T(7))

    return a.foo(Array(b))
        /* Ok! */
}
let myInt2d = bar2(arr1d, arr2d) // 10, OK
let myInt2my = bar2(arr1my, arr2my) // 10, OK

How can I extend Swift's Array<T> or T[] type with custom functional utils?

Browsing around Swift's API docs shows that Array methods are an extension of the T[], e.g:

extension T[] : ArrayType {
    //...
    init()

    var count: Int { get }

    var capacity: Int { get }

    var isEmpty: Bool { get }

    func copy() -> T[]
}

When copying and pasting the same source and trying any variations like:

extension T[] : ArrayType {
    func foo(){}
}

extension T[] {
    func foo(){}
}

It fails to build with the error:

Nominal type T[] can't be extended

Using the full type definition fails with Use of undefined type 'T', i.e:

extension Array<T> {
    func foo(){}
}

And it also fails with Array<T : Any> and Array<String>.

Curiously Swift lets me extend an untyped array with:

extension Array {
    func each(fn: (Any) -> ()) {
        for i in self {
            fn(i)
        }
    }
}

Which it lets me call with:

[1,2,3].each(println)

But I can't create a proper generic type extension as the type seems to be lost when it flows through the method, e.g trying to replace Swift's built-in filter with:

extension Array {
    func find<T>(fn: (T) -> Bool) -> T[] {
        var to = T[]()
        for x in self {
            let t = x as T
            if fn(t) {
                to += t
            }
        }
        return to
    }
}

But the compiler treats it as untyped where it still allows calling the extension with:

["A","B","C"].find { $0 > "A" }

And when stepped-thru with a debugger indicates the type is Swift.String but it's a build error to try access it like a String without casting it to String first, i.e:

["A","B","C"].find { ($0 as String).compare("A") > 0 }

Does anyone know what's the proper way to create a typed extension method that acts like the built-in extensions?


Using Swift 2.2: I ran into a similar issue when trying to remove duplicates from an array of strings. I was able to add an extension on the Array class that does just what I was looking to do.

extension Array where Element: Hashable {
    /**
     * Remove duplicate elements from an array
     *
     * - returns: A new array without duplicates
     */
    func removeDuplicates() -> [Element] {
        var result: [Element] = []
        for value in self {
            if !result.contains(value) {
                result.append(value)
            }
        }
        return result
    }

    /**
     * Remove duplicate elements from an array
     */
    mutating func removeDuplicatesInPlace() {
        var result: [Element] = []
        for value in self {
            if !result.contains(value) {
                result.append(value)
            }
        }
        self = result
    }
}

Adding these two methods to the Array class allows me to call one of the two methods on an array and successfully remove duplicates. Note that the elements in the array must conform to the Hashable protocol. Now I can do this:

 var dupes = ["one", "two", "two", "three"]
 let deDuped = dupes.removeDuplicates()
 dupes.removeDuplicatesInPlace()
 // result: ["one", "two", "three"]

For extending typed arrays with classes, the below works for me (Swift 2.2). For example, sorting a typed array:

class HighScoreEntry {
    let score:Int
}

extension Array where Element:HighScoreEntry {
    func sort() -> [HighScoreEntry] {
      return sort { $0.score < $1.score }
    }
}

Trying to do this with a struct or typealias will give an error:

Type 'Element' constrained to a non-protocol type 'HighScoreEntry'

Update:

To extend typed arrays with non-classes use the following approach:

typealias HighScoreEntry = (Int)

extension SequenceType where Generator.Element == HighScoreEntry {
    func sort() -> [HighScoreEntry] {
      return sort { $0 < $1 }
    }
}

In Swift 3 some types have been renamed:

extension Sequence where Iterator.Element == HighScoreEntry 
{
    // ...
}

I had a look at the Swift 2 standard library headers, and here is the prototype for the filter function, which makes it quite obvious how to roll your own.

extension CollectionType {
    func filter(@noescape includeElement: (Self.Generator.Element) -> Bool) -> [Self.Generator.Element]
}

It's not an extension to Array, but to CollectionType, so the same method applies to other collection types. @noescape means that the block passed in will not leave the scope of the filter function, which enables some optimisations. Self with a capital S is the class we are extending. Self.Generator is an iterator that iterates through the objects in the collection and Self.Generator.Element is the type of the objects, for example for an array [Int?] Self.Generator.Element would be Int?.

All in all this filter method can be applied to any CollectionType, it needs a filter block which takes an element of the collection and returns a Bool, and it returns an array of the original type. So putting this together, here's a method that I find useful: It combines map and filter, by taking a block that maps a collection element to an optional value, and returns an array of those optional values that are not nil.

extension CollectionType {

    func mapfilter<T>(@noescape transform: (Self.Generator.Element) -> T?) -> [T] {
        var result: [T] = []
        for x in self {
            if let t = transform (x) {
                result.append (t)
            }
        }
        return result
    }
}

If you want to learn about extending Arrays and other types of build in classes checkout code in this github repo https://github.com/ankurp/Cent

As of Xcode 6.1 the syntax to extend arrays is as follows

extension Array {
    func at(indexes: Int...) -> [Element] {
        ... // You code goes herer
    }
}

import Foundation

extension Array {

    func calculateMean() -> Double {
        // is this an array of Doubles?
        if self.first is Double {
            // cast from "generic" array to typed array of Doubles
            let doubleArray = self.map { $0 as! Double }

            // use Swift "reduce" function to add all values together
            let total = doubleArray.reduce(0.0, combine: {$0 + $1})

            let meanAvg = total / Double(self.count)
            return meanAvg

        } else {
            return Double.NaN
        }
    }

    func calculateMedian() -> Double {
        // is this an array of Doubles?
        if self.first is Double {
            // cast from "generic" array to typed array of Doubles
            var doubleArray = self.map { $0 as! Double }

            // sort the array
            doubleArray.sort( {$0 < $1} )

            var medianAvg : Double
            if doubleArray.count % 2 == 0 {
                // if even number of elements - then mean average the middle two elements
                var halfway = doubleArray.count / 2
                medianAvg = (doubleArray[halfway] + doubleArray[halfway - 1]) / 2

            } else {
                // odd number of elements - then just use the middle element
                medianAvg = doubleArray[doubleArray.count  / 2 ]
            }
            return medianAvg
        } else {
            return Double.NaN
        }

    }

}




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