Say I declare a `fun interface Foo` and I have `cl...

# getting-started

[dave08]

Say I declare a fun interface Foo and I have class Bar(val foo: Foo) I can still create Bar using a lambda that doesn't implement the fun interface... but when I try to do fun provideFoo(): Foo = { ... }, it doesn't let me... why not? What's the difference? Can I maybe cast it to work somehow without wrapping it in a Foo { { }() }?

[Sam]

https://kotlinlang.slack.com/archives/C0B9K7EP2/p1675252331980199 😉

[ephemient]

using a lambda that doesn't implement the fun interface

that's not true, the lambda in that context does implement the fun interface. if you write

val foo: (...) -> ... = { ... }

there is no automatic conversation that lets you write

Bar(foo)

[ephemient]

only trailing lambdas get SAM converted without specifying the interface type

[dave08]

It seems like this works:

val someLambda = { ... }

fun provideFoo(): Foo = Foo(someLambda)

I wonder if that syntax is documented somewhere...

[ephemient]

e.g.

Bar { ... }

is really

Bar(Foo { ... })

[ephemient]

yes, that was added sometime around 1.6

[ephemient]

you end up with an extra wrapper lambda object that way though

[dave08]

😕

[dave08]

At least I don't have to rewrite all the params and pass them down to the other lambda... it's not even really a lambda but rather a function reference.

[ephemient]

https://kotlinlang.org/docs/fun-interfaces.html#migration-from-an-interface-with-constructor-function-to-a-functional-interface

[dave08]

My fun interface represents a use case, and I'm just calling the repository's function that implements it instead of giving it access to the repo... but it doesn't make sense for the repo to implement the use case, since it's in a different layer.

[dave08]

I saw that example but I don't understand what ::Printer does... you're calling a function reference on the actual fun interface ...?

[dave08]

What's the declaration of that addPrinter method?

[ephemient]

https://kotlinlang.slack.com/archives/C0B8MA7FA/p1680175782037219?thread_ts=1680174463.298779&cid=C0B8MA7FA

[Javier]

I think this is working in K2

[dave08]

I still don't understand what ::Printer is instantiating, it's just an interface w/o an implementation?

[ephemient]

it is referencing the

(() -> Unit) -> Printer

"constructor" which you used to need to write by hand but is now provided by the Kotlin compiler