Whats the policy on extension functions on code I own Say I

Question

Whats the policy on extension functions on code I own Say I have `data class PhoneNumber val normalized String ` and I need to format it with spaces etc before rendering in UI Is this a member or extension function

Stephan Schroeder · Answer

afaik there are different schools of thought on this Classically it d be a member function but I like the idea of only the core functionality should be in the class itself so I d probably use an extension function at least in my own projects since my colleagues at work have only recently started with Kotlin so in order to have more harmonious code I d probably lower my code style a bit to classical

Stephan Schroeder · Answer

oh by the way since you only have a single property you should probably use ` JvmInline value class PhoneNumber val normalized String ` since this would avoid the wrapper overhead while retaining the type safety That only works on the JVM of course For other backends `data class` is fine More info <https kotlinlang org docs inline classes html>

ursus · Answer

Yes I also like the extension however I think my key would be if formatting was in another module i e you only need to format in ui so ui includes formatter module

Daniel · Answer

This might be a usecase for a regular `PhoneNumberFormatter` class This way the formatting is nicely encapsulated and you can mock it in unit tests if need be

Daniel · Answer

When new to Kotlin some1 tends to write too many extension functions since they are very cool So best decide first if it is really a good case for an extension

Daniel · Answer

A good thought for extension no extension is the following Is the possible extension something general or domain specific General lends itself more to extensions like `isNotEmpty` and domain specific lends itself more to member functions or other classes

Jacob · Answer

If every client of the class will need the function then IMO member is the way to go

gildor · Answer

I almost always use extensions for data classes it doesn t matter general or domain specific I try to avoid mixing data structure with business logic for it

gildor · Answer

And imo PhoneNumber it s a perfect example where you shouldn t use member function for such flexible and domain specific thing as number formatting number formatting depends on language and region it may depends on user s own preferences etc so number formatting shouldn t be backed to PhoneNumber and instead you need a separate component class which can work with PhoneNumber formatting