Functions as Values

Functions have types in Go and can be assigned to variables, passed as arguments and returned as results. This helps separate an operation from the logic that chooses when to run it.

A function type records parameter and result types, such as func(int) int. Any function with the matching signature can be passed where that type is expected. A higher-order function accepts another function or returns one.

In the example, apply does not care how square computes its result; it simply calls the received operation. This is useful for transformations, validation rules and callbacks.

Follow this diagram before executing the example. Check where a value or decision changes what runs next.

This example is a complete program. Save it as main.go or paste it into the Go Playground.

Go code

package main

import "fmt"

func square(n int) int { return n * n }
func triple(n int) int { return n * 3 }
func apply(value int, operation func(int) int) int {
	return operation(value)
}
func main() {
	fmt.Println("Square:", apply(5, square))
	fmt.Println("Triple:", apply(5, triple))
}

Output

Square: 25
Triple: 15

The first apply call receives the function value square and evaluates square(5) to 25. The next receives triple and returns 15. Passing square without parentheses passes the function, rather than calling it first.

• square is a function value, while square(5) is a call producing an integer.
• Parameter and result signatures must be compatible when assigning or passing function values.

Add a decrement function and pass it into apply. Predict the output for input 5.

Tip: Edit the example above, predict the output, and run the modified code to test your reasoning.

You have practiced Functions as Values in a small, self-contained program. The important habit is to predict what runs, inspect the actual output and distinguish the language rule from the particular example.

Before continuing, make sure you can explain the program to someone who has not seen it, including the line that changes its result.