what s the syntax for checking type and then using typed obj kotlinlang #announcements

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Title

twisterrob

08/14/2018, 7:58 PM

what's the syntax for checking type and then using typed object? kind of like a

.let<Type> { }

val something: X
fun f() {
	if (something is Y) {
		(something as Y).g()
	}
}

I see two options:

val something: X
fun f() {
	val cacheOfSomething = something
	if (cacheOfSomething is Y) {
		(cacheOfSomething as Y).g()
	}
}

(something as? Y)?.g()

neither of which is pleasing, any other way to do this?

adam-mcneilly

08/14/2018, 7:59 PM

If you have

(something is Y)

you should be able to smartcast inside of that block, no?

👆 3

dknapp

08/14/2018, 7:59 PM

you don't need to cast it with as if you are in the scope of the if x is y check

adam-mcneilly

08/14/2018, 7:59 PM

if (thing is String) { println(thing.length) }

➕ 2

twisterrob

08/14/2018, 8:34 PM

oh, sorry, the val is outside the scope of the funciton, it could change value over time (custom getter) the

as Y

is not showing as redundant

adam-mcneilly

08/14/2018, 8:35 PM

ooo

Well sometimes I'll do

(something as? MyClass)?.let { ... }

which would work in your example I believe

trevjones

08/14/2018, 9:35 PM

something.takeIf { it is Y }?.let { it.g() }

😬 1

adam-mcneilly

08/14/2018, 9:36 PM

Robert, what about

(something as? Y)?.g()

wasn't preferable to you? I think it's the most idiomatic way to accomplish what you're trying to do.

trevjones

08/14/2018, 9:42 PM

or i guess drop the let block.

something.takeIf { it is Y }?.g()

one bonus of using

is

is that the ide can tell you the check for instance is always true/false.

twisterrob

08/14/2018, 10:14 PM

@trevjones I like

takeIf

, but I'm trying to call

Y.g()

, not

X.g()

😞 (

takeIf

is not aware of cast) @adam-mcneilly it introduces null to the mix, even though there's nothing nullable going on in the expression, it's just a simple non-null typecheck+cast.

trevjones

08/14/2018, 10:18 PM

perhaps this:

inline fun <reified T, R: Any> Any?.takeAs(crossinline block: (T) -> R?): R? {
  return if (T::class.java.isInstance(this)) block(this as T) else null
}

something.takeAs<Y> { it.g() }

twisterrob

08/14/2018, 10:24 PM

yep, just got to a similar conclusion:

inline fun <reified T> Any.run(crossinline block: T.() -> Unit) {
	if (this is T) { block(this) }
}

note: you can do

this is T

on reified no need for explicit reflection.

trevjones

08/14/2018, 10:25 PM

yea I just like the

isInstance

because I can hit F1 on it and get a spew of documentation

🤔 1

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