lambda

In Kotlin, a lambda is an anonymous function that can be treated as a value.

// CharSequence.count(predicate: (Char) -> Boolean)
"A link to the past".count({ letter -> letter == 't' }) // 3

// We can remove the `()` when the last argument is a function
println("A link to the past".count {
  letter -> letter == 't'
})

/*
** When using lambda with a single argument,
** you can either give the argument a name or use the `it` keyword
** as a shorthand way to refer to the argument.
*/
"A link to the past".count { it == 't' } // 3

💡 A return inside a lambda expression will return from the enclosing function.

Syntax

// The value of the last expression is implicitly returned.
val sum: (Int, Int) -> Int = { x: Int, y: Int -> x + y }

// Removing optional annotations out
val sum = { x: Int, y: Int -> x + y }

Anonymous functions

val nums = arrayListOf(1,2,3,4,5)
nums.filter(fun(item) = item > 3)) // [4, 5]

💡 A return statement without a label always returns from the function declared with the fun keyword.

Closures

Kotlin functions can access its closure, which includes the variables declared in the outer scope.

val nums = arrayListOf(1,2,3,4,5)

var sum = 0
nums.filter { it > 0 }.forEach {
    sum += it
}
print(sum) // 15