First-class functions

Learn to pass functions as values.

First-class functions

In Dart functions are values: they can be assigned to a variable, passed as an argument, or returned. This is called being first-class.

Example: int Function(int) f = (n) => n * 2;. Here f holds a function; calling f(5) gives 10. The type int Function(int) reads 'function taking int, returning int'.

When a function is passed as an argument it is called a callback. For example apply(f, 5) takes a function and a value, applying the function to the value. This is the foundation of callbacks and higher-order functions.

Callbacks and higher-order

A function that takes another function as a parameter is called higher-order. Example: void apply(int Function(int) f, int x) { print(f(x)); }. It applies f to x and prints the result.

The function passed in is a callback. The caller invokes it when its work is done. List.map, List.forEach and similar Dart methods accept callbacks.

This pattern makes it easy to swap pieces: pass a different f to reuse the same apply. That cuts code repetition.

Exercise The apply function

Define an apply higher-order function: void apply(int Function(int) f, int x). Its body should do print(f(x)). Add an apply((n) => n * 3, 5) call inside main.

Show example solution

void apply(int Function(int) f, int x) {
  print(f(x));
}

void main() {
  apply((n) => n * 3, 5);
}

Test yourself

What does int Function(int) mean?
- A function returning int
- Two int params
- A function taking int and returning int
- A list of int
Show answer

A function taking int and returning int — int Function(int) is the type of a function taking int and returning int.
Match the first-class concepts

int Function(int)

Function type
(n) => n * 2

Anonymous function
f(5)

Function call
apply(f, 5)

Higher-order call

The function type

var f = (n) => n * 2;
int Function(int) g = ____;

Show answer

var f = (n) => n * 2;
int Function(int) g = f;

Next lesson: Summary - Functions

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