Basic Lambdas

Lambda as parameter to filter function

val allowedUsers = users.filter { it.age > MINIMUM_AGE }

Lambda passed as a variable

val isOfAllowedAge = { user: User -> user.age > MINIMUM_AGE }
val allowedUsers = users.filter(isOfAllowedAge)

Lambda for benchmarking a function call

General-purpose stopwatch for timing how long a function takes to run:

object Benchmark {
    fun realtime(body: () -> Unit): Duration {
        val start = Instant.now()
        try {
            body()
        } finally {
            val end = Instant.now()
            return Duration.between(start, end)
        }
    }
}

Usage:

val time = Benchmark.realtime({
    // some long-running code goes here ...
})
println("Executed the code in $time")

Syntax

Explicit parameters:

{ parameterName: ParameterType, otherParameterName: OtherParameterType -> anExpression() }

Inferred parameters:

val addition: (Int, Int) -> Int = { a, b -> a + b }

Single parameter `it` shorthand

val square: (Int) -> Int = { it*it }

Signature:

() -> ResultType

(InputType) -> ResultType

(InputType1, InputType2) -> ResultType

Remarks

Input type parameters can be left out when they can be left out when they can be inferred from the context. For example say you have a function on a class that takes a function:

data class User(val fistName: String, val lastName: String) {
    fun username(userNameGenerator: (String, String) -> String) =
        userNameGenerator(firstName, secondName)
}

You can use this function by passing a lambda, and since the parameters are already specified in the function signature there’s no need to re-declare them in the lambda expression:

val user = User("foo", "bar")
println(user.userName { firstName, secondName ->
     "${firstName.toUppercase}"_"${secondName.toUppercase}"
 }) // prints FOO_BAR

This also applies when you are assigning a lambda to a variable:

//valid:
val addition: (Int, Int) = { a, b -> a + b }
//valid:
val addition = { a: Int, b: Int -> a + b }
//error (type inference failure):
val addition = { a, b -> a + b }

When the lambda takes one parameter, and the type can be inferred from the context, you can refer to the parameter by it.

listOf(1,2,3).map { it * 2 } // [2,4,6]