Could someone please tell me why I get a `Type mis...

# announcements

[Ellen Spertus]

Could someone please tell me why I get a Type mismatch error in this code?

private fun applyIfPossible(contacts: List<ContactEntity>): Unit {
    contacts.firstOrNull() ?.run { contact ->
        println(contact)
    }
}

[Shawn]

run is scoped such that the context object in the lambda it accepts is this, i.e. T.() rather than (T), or accepts T

[Shawn]

if you pass it a lambda with a named parameter, the compiler can’t figure out what you mean

[Shawn]

if you get rid of contact -> and instead println(this), it’ll work

[Ellen Spertus]

You’re right, it does.

[Ellen Spertus]

I wasn’t expecting explicitly naming a variable to break something.

[Casey Brooks]

.let{ } would be equivalent to what you’re wanting (a named parameter passed to the lambda)

[Shawn]

I think you expected something more like let or also here

[Casey Brooks]

The confusion here is that the receiver isn’t really a parameter of the lambda. It’s a variable that’s “passed to” the lambda, but the syntax and semantics of a receiver are very different from a parameter, and they are not interchangeable

[Ellen Spertus]

Yes, I don’t fully grok the differences among let, also, run, etc. I made myself a table of whether they use `it`/`this` and what they return, but the differences seem to run deeper.

[Ellen Spertus]

Thanks, @Shawn and @Casey Brooks. Are there any books or articles you recommend to help me wrap my head around this?

[Shawn]

uhh there’s a flow chart floating around here somewhere…

[Ellen Spertus]

@Shawn I don’t expect you to do research for me, just wondering if there’s some source you learned these aspects of Kotlin from that you’d recommend.

[Shawn]

here’s the medium article most folks seem to like https://medium.com/androiddevelopers/kotlin-demystified-scope-functions-57ca522895b1

[Shawn]

though you’ve probably seen it already 😅

[Shawn]

someone also made this

[Shawn]

though that chart is a bit inaccurate

[Shawn]

or, rather

[Shawn]

it doesn’t plainly state that run is overloaded

[Shawn]

that’s really the trickiest part of this all tbh

[Ellen Spertus]

Right, that’s what got me.

[Shawn]

I don’t expect you to do research for me

no, totally, I get what you’re saying and I’m happy to dig into the archives a bit for someone making a good faith attempt to learn lol

[Shawn]

wrt the notion of there being a source for the stuff I’ve learned, honestly, I couldn’t give you a better answer than just through experience and idling on here

[Ellen Spertus]

Makes sense. Thanks again for the help!

[rocketraman]

The article I found most helpful to understand these was from @cedric: http://beust.com/weblog/2015/10/30/exploring-the-kotlin-standard-library/

[rocketraman]

And a prerequisite to that is really understanding functions with receivers: https://kotlinlang.org/docs/reference/lambdas.html#function-literals-with-receiver

[Alexey Belkov [JB]]

Now we have a definitive guide on this topic: https://kotlinlang.org/docs/reference/scope-functions.html