Generic functions

Learn generic functions with fun <T>.

Generic function

A generic function takes a type parameter. It is declared as fun <T> ident(t: T): T; the type parameter is written before the function name.

Example: fun <T> ident(t: T): T { return t }. This function is called as ident<Int>(5) or ident("hello"). In the second call Kotlin infers the type; T becomes String. Generic functions let you write type-safe helpers. The type parameter is written BEFORE the function name; this is an important detail.

Type inference

When a generic function is called, the type parameter is usually inferred. Writing it explicitly is optional.

Example: fun <T> singleton(value: T): List<T> { return listOf(value) }. In singleton(5) Kotlin infers T as Int; the result is List<Int>. You can also write singleton<String>("hello") explicitly. Type inference keeps code short; you can still write the explicit type when it helps readability. The compiler checks type safety in both cases.

Exercise The ident generic function

Define the generic function fun <T> ident(t: T): T whose body returns t. In main call println(ident(42)) and println(ident("hello")) on separate lines.

Show example solution

fun <T> ident(t: T): T {
    return t
}

fun main() {
    println(ident(42))
    println(ident("hello"))
}

Test yourself

Match the generic function parts

fun <T> ident

Type-parameterized signature
(t: T)

Type-parameterized parameter
: T

Return type
ident<Int>(5)

Call
Where is the type parameter written in a <strong>generic function</strong> declaration?
- After the return type
- After the function name
- At the end of the parameter list
- Before the function name, with fun
Show answer

Before the function name, with fun — The type parameter is written before the name, as in fun <T> ident.

Complete the generic function definition

fun ____ ident(t: T): T {
    return t
}

Show answer

fun <T> ident(t: T): T {
    return t
}

Next lesson: Variance

We Are Enhancing Your Experience

Cookie Preferences

Strictly Necessary Cookies

Analytical Cookies