What is a functor?
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.
Available from iOS 8.0 – learn more in my book Pro Swift
Did this solution work for you? Please pass it on!
Other people are reading…
- Private vs fileprivate: what’s the difference?
- How to find the maximum of two numbers
- How to convert a Substring to a String
- What is the ternary operator?
- How to find the difference between two arrays
About the Swift Knowledge Base
This is part of the Swift Knowledge Base, a free, searchable collection of solutions for common iOS questions.
Need to know Objective-C fast?
I wrote a book dedicated to teaching Objective-C to developers who already know Swift – it's the fastest way to get up to speed!
