Hi :wave: - What is the correct way to write a function signature using generics, whereby the first ...

# getting-started

[James Whitehead]

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 🙂

[Joffrey]

For your first question, do you mean "T or a subtype of T"?

I guess this should work:

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

}

[Joffrey]

For the second question, <T> denotes any type including nullable types, while <T : Any> forbids nullable types for T, because nullable types are not subtypes of Any

[James Whitehead]

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?

[James Whitehead]

Thank you, second answer makes perfect sense to me now!

[Joffrey]

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.

[Jacob]

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

[James Whitehead]

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

[Joffrey]

@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

[Matteo Mirk]

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