Hey guys :wave: I'm trying context parameters, but...

# getting-started

[Rafael Costa]

Hey guys 👋
I'm trying context parameters, but failing in this case:

fun bar(
    lambda: context(String) () -> Unit
) {
    with("") {
        lambda()
    }
}

fun foo() {
    bar {
        this // error
    }
}

Am I missing something? Is this not supposed to give me a string?
I am not able to get the context parameter of the lambda in any way. If I try bar { str -> then "str" here is also an error 🤔

[Joffrey]

this is no longer the correct syntax to access context receivers, you should now use implicit<...>(). See the KEEP: https://github.com/Kotlin/KEEP/blob/context-parameters/proposals/context-parameters.md#standard-library-support

[Rafael Costa]

I don't have this std library function 🤔
I'm applying kmp plugin with version 2.2.0, so it should also automatically add standard library dependency for that same version, right?

[Rafael Costa]

And also, shouldn't I be able to just call methods on String type directly?
Maybe string is confusing here, but this next example:

interface Logger {
    fun logme(msg: String)
}

fun bar(
    lambda: context(Logger) () -> Unit,
) {
    val logger = object : Logger {
        override fun logme(msg: String) = println(msg)
    }
    with(logger) {
        lambda()
    }
}

fun foo() {
    bar {
        logme("asd")
    }
}

[Rafael Costa]

I would expect logme to work, no? 🤔

[Joffrey]

Ah maybe the implicit function wasn't implemented 🤔

[Joffrey]

> I would expect logme to work, no? 🤔
No, you would need to refer to the parameter, which is not acccessible in the lambda. logme is defined with Logger as receiver. If you define another logme function with context parameter, it would work:

context(logger: Logger)
fun logme(msg: String) = logger.logme(msg)

[Joffrey]

In general you should use context parameters to forward the context implicitly, but when you reach the final place where you actually use the context parameter, you need to refer to it by name. This is the major difference with the original context receivers proposal

[Youssef Shoaib [MOD]]

I thought the function was called contextOf? I can see it in Kotlin 2.2 projects

[Rafael Costa]

I see now, thanks @Joffrey

[Rafael Costa]

This means that if I want my function to enable users to just call `logme`I need to create a kind of wrapper function that calls the original one. This one would then need to be imported as well.

It's a little annoying, but I guess it works 😛
Should probably read the KEEP more thoroughly to understand why they did it like this.

[Rafael Costa]

And this "annoyance" is only for this use case of the lambda with context parameter, since it is forcedly unnamed, so users have to use implicit() .
For named params case, I think it's fine to just use the name 🙂