Where does one use Kotlin anonymous functions defined using the fun keyword? Below is an example taken from their documentation. How would one go about invoking this or passing this into function / object, etc.
fun(s: String): Int { return s.toIntOrNull() ?: 0 }
Anonymous functions provide an alternate syntax to pass a lambda, so both the following lines produce identical result
intList.filter(fun(item: Int): Boolean { return item > 0 })
intList.filter{ it > 0 }
to demonstrate this, Lets consider a trivial lambda which contains multiple return statements
strList.forEachIndexed { index, s ->
when(index){
0 -> return@forEachIndexed
1 -> return@forEachIndexed
3 -> return@forEachIndexed
else -> { Log.d("TAG", "$index -> $s")}
}
}
here if we used return instead of return@forEachIndexed, that would return us from the outer scope. to return from lambda, we need to specify the label.
If however the lambda contains multiple return@forEachIndexed statements then this syntax becomes very redundant and verbose, it would be nice if we could use simple return without that label. this is where anonymous functions come in.
using anonymous function above code can be written as
strList.forEachIndexed(fun (index, string) {
when(index){
0 -> return // No need to specify label, this returns from anon function not from outer scope
1 -> return
3 -> return
else -> { Log.d("TAG", "$index -> $string")}
}
})
From the same documentation:
The parameters and the return type are specified in the same way as for regular functions, except the parameter types can be omitted if they can be inferred from the context:
ints.filter(fun(item) = item > 0)
In practice:
fun main() {
val ints = listOf(-2, -1, 0, 1, 2)
// As in your snippet
println(ints.filter(fun(item: Int): Boolean { return item > 0 }))
// Outputs [1, 2]
// Or simply
println(ints.filter(fun(item) = item > 0))
// Outputs [1, 2]
}
fun in these use cases vs. using a lambda? Presumably there is no use case then of standalone anonymous function definitions (i.e. where a lambda is not feasible).
