Poly is a small library to provide an alternative to rolling your own type-erasure when a value has one of a small set of Types. The Poly library contains the Types Poly1
, Poly2
, Poly3
, etc. for representing increasingly larger pools of possible Types. Poly2
is isomorphic to Either
(a common generic functional programming Type).
- Swift 4.2+
- Swift Package Manager 5.0 OR Cocoapods
To use this framework in your project via Cocoapods instead of Swift Package Manager, add the following dependency to your Podfile.
pod 'Poly', :git => 'https:/mattpolzin/Poly.git'
To create an Xcode project for Poly, run
swift package generate-xcodeproj
Usage will be explained by way of an example. Suppose you have some code with three different Types: Dog
, Cat
, and Rat
. You also have a protocol, Animal
, that they all belong to.
If you need to store animals of all three Types in one place (maybe an array), that looks like:
let dog = Dog()
let cat = Cat()
let rat = Rat()
let animals: [Poly3<Dog, Cat, Rat>] = [
.init(dog),
.init(cat),
.init(rat)
]
To access all animals of a certain type, you can use subscripting like:
let dogs = animals[Dog.self]
let cats = animals[Cat.self]
let rats = animals[Rat.self]
You can get the Dog
, Cat
, or Rat
value back out again, but you won't get any guarantees of which Type is being stored in a given Poly
:
let animal = Poly3<Dog, Cat, Rat>(Dog())
let maybeDog: Dog? = animal.a
let maybeCat: Cat? = animal.b
let maybeRat: Rat? = animal.c
Or use the subscript operator to make accessing one of the possible values of a Poly
a bit more intuitive:
let maybeDog2 = animal[Dog.self]
let maybeCat2 = animal[Cat.self]
let maybeRat2 = animal[Rat.self]
Or switch over the possible values:
switch animal {
case .a(let dog):
print(dog)
case .b(let cat):
print(cat)
case .c(let rat):
print(rat)
}
Or access a type-erased value:
let someAnimal: Any = animal.value
You might consider making a typealias to make your life easier:
typealias AnyAnimal = Poly3<Dog, Cat, Rat>
You also might find it worthwhile to go the extra mile and add Animal
conformance to Poly<Dog, Cat, Rat>
:
protocol Animal {
var speak: String { get }
}
extension Poly3: Animal where A == Dog, B == Cat, C == Rat {
var speak: String {
switch self {
case .a(let animal as Animal),
.b(let animal as Animal),
.c(let animal as Animal):
return animal.speak
}
}
}
So now you can take the array of animals from the first example above and:
let animalSounds = animals.map { $0.speak }
All of the Poly
types are Encodable
/Decodable
when every generic on which they are specialized is Ecodable
/Decodable
. This works by attempting to encode or decode each type and using the first successfuly attempt. That means the behavior only works as expected if all of the types have different encoding/decoding requirements or at least types are ordered with more restrictive rules coming first.
For example, given the following type
struct Type1: Decodable {
let x: Poly2<Double, Int>
}
an integer value will never be decoded as an Int
because Double
is capable of decoding all Int
values.
You can fix this by swapping the order to Poly2<Int, Double>
. Now Poly
will attempt to decode an Int
and only attempt to decode a Double
if an Int
was not found.