Hi 👋 • What is the correct way to write a function signature using generics, whereby the first parameter is something like 

<T>

 and the second parameter is "an instance or implementation of T"? • Also kind of related question: When should one use

<T>

vs

<T : Any>

, I'm not really sure what the difference is? Thanks in advance 🙂

For your first question, do you mean "T or a subtype of T"? I guess this should work:

fun <T, U : T> someFunction() {

}

Yes that's right, "T or a subtype of T" is what I was going for 🙂 Is there a convention for knowing which letter to use for a generic type? I.e. In your example you have used

U

, how did you know to use that particular letter?

The letters are arbitrary, it's like variable names. The usual convention is to use single letters, but sometimes even whole words are used.

T

is a very classic example, because it's T as in "Type".

U

is simply the next letter in the alphabet. Sometimes other letters make sense. For instance

K

and

V

for the types of the keys and values in a map, or

E

for the element type in a collection.

Letters are arbitrary but only under VERY special circumstances should you EVER use a more than one char name for a type parameter. Breaking the one char convention can make things get really confusing

Thank you very much @Joffrey and @Jacob makes much more sense to me now 🙂 I used to be PHP dev so generics are something I'm still getting my head around after transitioning to Java/Kotlin

@Jacob yes, single letters are a very common convention indeed, but I've seen 2-letter names used quite regularly too. That said I would also strongly suggest to stick to single letters, unless you have a good reason to do differently. Sometimes it may be interesting to use a different convention if you're trying to match a scientific paper's names for an algorithm you're implementing, or if your types represent things with the same initial and you need a second letter

I’ve seen some codebases using a whole WORD as a type identifier, such as Assertj