Function types

Learn function types with (Int) -> String.

Function type

A function type expresses a function's signature as a type. It is written as (Int) -> String; parameter types go in parentheses, the return type after the arrow.

Example: val format: (Int) -> String = { n -> "Number: $n" }. Here format is a function that takes an Int and returns a String. Calling format(5) returns "Number: 5". Function types let you carry functions as values. This is the foundation of higher-order functions.

The format function

A function type variable gets a lambda or a function reference. It is called by the variable name.

Example: val format: (Int) -> String = { n -> "Number: $n" }. Calling format(5) returns "Number: 5". n is the lambda parameter; its type is inferred as Int. A function type lets you pass functions as parameters or keep them in lists. You can also declare listOf<(Int) -> String>.

Exercise The format function type

Declare val format: (Int) -> String = { n -> "Number: $n" }. In main call println(format(5)).

Show example solution

val format: (Int) -> String = { n -> "Number: $n" }

fun main() {
    println(format(5))
}

Test yourself

What does the type <code>(Int) -> String</code> express?
```
In a function type, -> marks the return type.
```
- A function taking Int and returning String
- A conversion from Int to String
- A pair of two Ints
- A function turning String into Int
Show answer

A function taking Int and returning String — (Int) -> String is a function type that takes Int and returns String.

Complete the function type definition

val format: (Int) ____ String = { n -> "Number: $n" }

Show answer

val format: (Int) -> String = { n -> "Number: $n" }

Match the function type parts

(Int) -> String

Function type
{ n -> ... }

Lambda body
format(5)

Call
"Number: 5"

Return value

Next lesson: Lambda

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