Expression body

Learn to write expression-body functions with =.

Expression body: short definition with =

Functions that return a single expression can be written shortly with = instead of a block body. fun double(x: Int): Int = x * 2 means the same as fun double(x: Int): Int { return x * 2 }.

fun double(x: Int): Int = x * 2
val result = double(7)  // 14

In this form return is not written; the right side is returned automatically. The return type can also be inferred: fun double(x: Int) = x * 2.

When to use = vs { }?

For short functions returning a single expression, = is more readable. When the body has multiple statements, a condition or a loop, use the { } block body. Both can give the same result; the choice is a matter of style.

// expression body
fun half(x: Int): Int = x / 2

// block body
fun half(x: Int): Int {
    val result = x / 2
    return result
}

The return type can be inferred in an expression body; in a block body return must be written explicitly.

Exercise Write double with an expression body

Define fun double(x: Int): Int = x * 2 (expression body). From main print the result with println(double(7)).

Show example solution

fun main() {
    println(double(7))
}

fun double(x: Int): Int = x * 2

Test yourself

Match definition form to role

fun f() = x

Expression body
fun f() { }

Block body
: Int

Explicit return type
return x

Return inside a block

Fill in the blank: expression body

fun double(x: Int): Int ____ x * 2

Show answer

fun double(x: Int): Int = x * 2

What does fun square(x: Int) = x * x return?
- Unit (no return type)
- String
- Throws an exception
- Int (inferred type)
Show answer

Int (inferred type) — Even without a written return type, x * x is Int so Int is returned.

Next lesson: Default parameters

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