Hey all I m trying to understand extension functions I see t

Question

Hey all I m trying to understand extension functions I see the value in adding extension methods for other library authors code However I m trying to figure out the reason for doing so in your own code I m sure there is a reason because I m seeing it a lot within the Ktor code base I m curious since the Ktor devs have access to their own code why they didn t just add them as regular methods on a class Would some one be able to shed some light on this Thank you

ephemient · Answer

I don t have anything ktor specific knowledge but a couple general cases with `inline fun` for higher order functions for `reified` types only static and extension functions work because they must be resolved statically for effectively `final` functions they don t go through virtual function resolution anyway so pick whichever organizes the code best

Ruckus · Answer

A good read from a while back by < elizarov>

Rob Elliot · Answer

Other reasons There are generic methods whose type signatures can only be done in extension functions e g <https github com Mahoney playground goos blob bf8cce396b751ca1b983001c35c38776adc0821d gradle build plugins extract plugin src either Either kt L59 L68> It makes for much nicer null handling `val x T = foo bar ` is a lot better than `val x T = foo run bar it ` or `val x T = if foo = null bar foo else null` or making `bar` accept a nullable parameter This is particularly true with chaining `x foo bar ` where `foo` and `bar` are both extension functions on a non null receiver can save you a lot of null types Even within a codebase there are places where subject verb object is a desirable word order or subject verb but where the verb and or the object are not appropriate to the core domain of the subject e g you might have a `data class` representing an entity Your data class should not know how to persist itself but it s quite nice to be able to call `entity save ` `save ` can then be an extension function on `Entity` which has access to your repository Of course you could just call `save entity ` but it s surprising how much more legible it can be changing the order and it can reduce your nesting quite a bit too

Rob Elliot · Answer

A favourite of mine is turning single arg constructors converters into extension functions I d much rather read `val status HttpStatus = statusStr toInt toHttpStatus ` than `val status HttpStatus = HttpStatus Integer parseInt statusStr ` The former lets me read left to right I start with a String turn it into an Int turn that into an HttpStatus The latter I have to hunt to the end for the input and expand out through two layers of nesting

Rob Elliot · Answer

And I can make it accept a null or invalid string trivially `val status HttpStatus = statusStr toIntOrNull toHttpStatus ` which is a lot easier than handling null without the extension functions

Muhammad Talha · Answer

Thanks for the thoughts all Really cool to see that extension function implications are much more than just attaching methods on classes you don t own