Understanding the need for Kotlin let

You need to use let if you want to chain function calls that aren't defined on the type you are chaining from.

Let's say the definition of your function was this instead:

// Not defined inside the Test class
fun giveMeFive(t: Test) {
    return 5
}

Now, if you have a nullable Test and want to call this function in a chain, you have to use let:

val x = test?.let { giveMeFive(it) }

The .let{} extension function in Kotlin:

1. Takes the object reference as the parameter on which .let{} is called.

2. Returns value of any non-primitive data-type which has been returned from with let{} function. By default, it returns undefined value of kotlin.Any class.

Declaration in package kotlin:

public inline fun <T, R> T.let(block: (T) -> R): R {
    return block(this)
}

Simple practical demonstration to see how .let{} extension function works in Kotlin.

Sample Code 1:-

val builder = StringBuilder("Hello ")
println("Print 0: $builder")

val returnVal = builder.let { arg ->
    arg.append("World")
    println("Print 1: $arg")

    "Done" // Returnning some string
}
println("Print 2: $builder")
println("Print 3: $returnVal")

Sample Code 1 Output:-

Print 0: Hello 
Print 1: Hello World
Print 2: Hello World
Print 3: Done

In Sample Code 1:

1. We created the final object of type StringBuilder with initialization value "Hello ".

2. In builder.let{}, the builder object reference will be passed to arg.

3. Here, the output Print 2: Hello World and Print 3: Hello World means that the builder and arg, both are pointing to the same StringBuilder object-reference. That's why they both print the same String value.

4. In the last line of .let{} function block, we are returning "Done" String value which will be assigned to returnVal.

*Hence, we get Print 3: Done output from returnVal.

Sample Code 2:-

val builder = StringBuilder("Hello ")
println("Print 0: $builder")
val returnVal = builder.let { arg ->
    arg.append("World")
    println("Print 1: $arg")
    arg.length
    }

println("Print 2: $builder")
println("Print 3: $returnVal") // Now return val is int = length of string.

Sample Code 2 Output:-

Print 0: Hello 
Print 1: Hello World
Print 2: Hello World
Print 3: 11

In Sample Code 2:

What's difference:

In the last line of .let{} function block, we are returning the Int value equals to the length of StringBuilder object arg whose value at this line of code is "Hello World". So length = 11 of type Int will be assigned to returnVal.

*Hence, we get Print 3: 11 as output from returnVal.

Also try these:-

• .run() {}
• .apply() {}
• with(obj) {}
• .also() {}

Happy Coding...

fun <T, R> T.let(f: (T) -> R): R = f(this)

.let block does not equal to in multiThreading

val x? = null

if(x == null) {
}

and

x?.let{
}

.let block is thread-safe