NEW: Learn SwiftUI with my free YouTube video series! >>

What is a functor?

Swift version: 5.0

Paul Hudson    @twostraws   

A functor is any data type that can be mapped over using map(), as long it abides by two laws:

  1. 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.
  2. 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.

LEARN SWIFTUI FOR FREE I wrote a massive, free SwiftUI tutorial collection, and also have a growing list of free SwiftUI tutorials on YouTube – get started today!

Available from iOS 8.0 – learn more in my book Pro Swift

Similar solutions…

About the Swift Knowledge Base

This is part of the Swift Knowledge Base, a free, searchable collection of solutions for common iOS questions.

MASTER SWIFT NOW
Buy Testing Swift Buy Practical iOS 12 Buy Pro Swift Buy Swift Design Patterns Buy Swift Coding Challenges Buy Server-Side Swift (Vapor Edition) Buy Server-Side Swift (Kitura Edition) Buy Hacking with macOS Buy Advanced iOS Volume One Buy Advanced iOS Volume Two Buy Hacking with watchOS Buy Hacking with tvOS Buy Hacking with Swift Buy Dive Into SpriteKit Buy Swift in Sixty Seconds Buy Objective-C for Swift Developers Buy Beyond Code

Was this page useful? Let us know!