What is a functor?
Swift version: 5.6
A functor is any data type that can be mapped over using map(), as long it abides by two laws:
- If your map function is the identity function (i.e. it just returns each element without transformation) then your input and output must be the same.
- If you compose function A and function B to make function C then use the result with
map(), the result should be the same as mapping first A then B individually.
Both of those laws sound obvious, but that doesn’t mean they aren’t important. Arrays are functors, sets are functors, optionals are functors, and more.
SPONSORED You know StoreKit, but you don’t want to do StoreKit. RevenueCat makes it easy to deploy, manage, and analyze in-app subscriptions on iOS and Android so you can focus on building your app.
Sponsor Hacking with Swift and reach the world's largest Swift community!
Available from iOS 8.0 – learn more in my book Pro Swift
Similar solutions…
- What are property observers?
- What is an escaping closure?
- How to add a custom initializer to a struct without losing its memberwise initializer
- How to use map() with an optional value
- How to convert JSON into Swift objects using Codable
About the Swift Knowledge Base
This is part of the Swift Knowledge Base, a free, searchable collection of solutions for common iOS questions.
