03 March 2021

Iterating Over a Generic Sequence in Swift

Here’s a goofy bit of generic programming I do all the time in Swift—ironically, it’s one thing C++ makes easier than Swift!

The use case is simple: I want a function to be able to accept a container (really a Swift Sequence) of any type, as long as its values are of the right type. For instance, I want it to be able to take a Set<String>, an Array<String>, or a Whatever<String>… I don’t care what the container type is.

This is conceptually easy enough (“I want a Sequence whose whose elements are Strings), but I have to look up the syntax every time:

func countChars<Seq>(someStrings: Seq) -> [Int]
    where Seq: Sequence, Seq.Element == String {
  return someStrings.map({ $0.count })

let strings = Set(["1", "22", "333"])
let chars = countChars(someStrings: strings)
print(chars) // > some ordering of [1, 2, 3]

And here’s what it looks like when writing an initializer instead of a normal function:

extension Dictionary {
  init<Seq>(mapping seq: Seq, to transformer: @escaping (Seq.Element) -> Value)
      where Seq: Sequence, Seq.Element == Key {
    self.init(uniqueKeysWithValues: seq.lazy.map { ($0, transformer($0)) })

let strings = Set(["1", "22", "333"])
let charCounts = Dictionary(mapping: strings, to: { $0.count })
print(charCounts) // some ordering of ["1": 1, "22": 2, "333": 3]

Here’s a demo of the above examples you can play around with.


Programming Apple