Is there any news on whether we'll get default arg...

# language-proposals

[Marc Knaup]

Is there any news on whether we'll get default arguments for generic parameters? 🙂
à la class Something<Context = DefaultContext> -> val something: Something

[Dico]

Would this work exactly like function parameters, with the exception that supertype declarations are not necessary?

class Something<Required, Extra = Unit>
val a: Something<String, Extra = Int>

class Something<Elem, Coll : Collection<Elem> = List<Elem>>

val a: Something<String, Set<String>>

[Marc Knaup]

I would love too see that 🙂
And +1 for the named generic arguments

[kevinmost]

Would love to see named + default type parameters just like function parameters. I wonder if there's a technical reason this can't be implemented?

[cedric]

I’m struggling to find a downside, probably more a matter of priority and resources I’d guess.

[ilya.gorbunov]

Note that after introducing named generic arguments, renaming generic parameters would be no longer a source-compatible change.

[Dico]

There is the option of having only indexed type parameters with default values, and not using parameters by name. That's not what the poster asked for in the end. I also struggle to see how useful it would really be. We generally shouldn't encourage high numbers of generic type arguments right?

[Marc Knaup]

Yeah, generic types shouldn't become too complicated.
In one of my cases where default parameters would really help I have a single generic parameter called context so that you can pass additional domain-specific data and functionality around in a type-safe way. But if you don't need that then you constantly have to specify something like <SomeEmptyContextIAmNotUsingAnyway>. So just defaulting that away would be awesome and remove a lot of visual clutter 🙂

[Marc Knaup]

It's fine to just haved it for indexed parameters for now as with few generic parameters you can easily reorder them and put the most-likely default ones to the end.
Having generic parameters on par with function parameters (named etc.) would be great long-term but can be considered separately.

[themishkun]

have you tried typealias SomethingDefault = Something<DefualtContext> -> val something: SomethingDefault?

[Marc Knaup]

That way I'd have to introduce dozens of aliases and the library user would still have to write something special all the time for something they don't care about.

[themishkun]

Java users wrote constructor overloads for decades 😃

[Dico]

kek