Welcome to the Treehouse Community

The Treehouse Community is a meeting place for developers, designers, and programmers of all backgrounds and skill levels to get support. Collaborate here on code errors or bugs that you need feedback on, or asking for an extra set of eyes on your latest project. Join thousands of Treehouse students and alumni in the community today. (Note: Only Treehouse students can comment or ask questions, but non-students are welcome to browse our conversations.)

Looking to learn something new?

Treehouse offers a seven day free trial for new students. Get access to thousands of hours of content and a supportive community. Start your free trial today.

iOS Closures in Swift First Class Functions Higher Order Functions

Make sure you return the result of applying the function argument on self

that's not what Im doing for sure

// Enter your code below

extension String {

func transform((String) -> String) -> String {

return self


1 Answer

Caleb Kleveter
Caleb Kleveter
Treehouse Moderator 37,862 Points

First off, it looks like you missed the parameter label for the method's closure parameter.

About your question on applying the closure to self. To apply a method or function to a variable means to pass the variable into the method/function as a parameter. For example, when you use the print function, you are applying that function to whatever you print with it:

print("Hello World")
// Applying `print` to the string, "Hello World"