Challenge: Create a Phone Number Lookup |

This is a companion discussion topic for the original entry at

In the solution to this challenge, you are passing in the closure rather than calling the method in the closure with an anonymous argument, i.e. rather than { convert(phoneNumberL $0 }

I believe I understand what is happening here, but I’d like to understand more deeply. Can you point me to other resources around implementing an operator in this way? Thank you!