Newbie (ish) question .... How do I do what the co...

# getting-started

[Edward Muller]

Newbie (ish) question ....
How do I do what the comment in that asks (or something similar) ?
Updated link: https://pl.kotl.in/QerMFeLbh

[Edward Muller]

maybeMunge is obviously contrived. But I am looking for a way to pass a closure to a mapNotNull()

[Edward Muller]

I tried to do something like..
fun User.maybeMunge(x : String): () -> Pair<String,Int>? {
but couldn't figure out how to pass that to users.mapNotNull()

[Paul Griffith]

https://pl.kotl.in/o68cqNVIT
Is this what you’re looking for?

[Edward Muller]

I was trying to do something like this ... which I know won't work....

data class User(val name: String, val age: Int)

fun User.maybeMunge(x: String): () -> Pair<String, Int>? {
    return { it ->
         if (it.name == "Ed") null else Pair(x + it.name, it.age)
    }
}

fun main() {
    val users = listOf(User("Ed", 2), User("Sally", 20))
    println(users.mapNotNull(User::maybeMunge("x")))
}

or 

data class User(val name: String, val age: Int)

fun User.maybeMunge(x: String): () -> Pair<String, Int>? {
    return { 
         if (this.name == "Ed") null else Pair(x + this.name, this.age)
    }
}

fun main() {
    val users = listOf(User("Ed", 2), User("Sally", 20))
    println(users.mapNotNull(User::maybeMunge("x")))
}

[Edward Muller]

it doesn't have to be a class method I guess.

[Edward Muller]

basically I have a sequence/list of T and want to provide a closure to mapNotNull for mapping them.

[Edward Muller]

the closure is important because I need to use the variable passed to the closure in the function being called during mapNotNull. I was looking for a way to do that with :: for whatever reason. Maybe that's where I am hung up.

[Edward Muller]

and the way I am doing it is fine w/o a closure.

[Paul Griffith]

It seems like you’re trying to get currying/partial application to work, which does get a bit awkward (and/or impossible). Easiest thing to do is, yeah, just embrace the lambdas and not worry about the syntax too much

[Edward Muller]

I am extending the class with the method because the actual implementation of the mapNotNull is tied very much to that class.

[Paul Griffith]

Sure, nothing wrong with using an extension method and calling that in the mapNotNull. It seems like you’re trying to avoid the “explicit” open lambda body in your mapNotNull. AFAIK, the only way to do that is with some kind of function literal like I have in my example

[Edward Muller]

thx!

[Paul Griffith]

You can infer the type on the “partial” if you make your lambda types explicit:

val users = listOf(User("Ed", 2), User("Sally", 20))
    val xed = { it: User -> it.maybeMunge("x") }
    println(users.mapNotNull(xed::invoke))

[Paul Griffith]

Or you can use a fun interface if you want to make the name after the double-colon something other than `invoke`:

fun interface UserMunger {
    fun munge(user: User): Pair<String, Int>?
}

fun main() {
    val users = listOf(User("Ed", 2), User("Sally", 20))
    val yed = UserMunger { it.maybeMunge("y") }
    println(users.mapNotNull(yed::munge))
}

[Edward Muller]

yeah, at that point I may as well just call users.mapNotNull { it.maybeMunge("x") }
Really I guess I am trying to do this:

data class User(val name: String, val age: Int)

fun maybeMunge(x: String): (User) -> Pair<String, Int>? {
    return { it ->
         if (it.name == "Ed") null else Pair(x + it.name, it.age)
    }
}

fun main() {
    val users = listOf(User("Ed", 2), User("Sally", 20))
    val mm = maybeMunge("x")
    println(users.mapNotNull(mm::invoke))
}

[Edward Muller]

which works

[Edward Muller]

not sure why I got so hung up on the extension way