Functions

Introduction#

Functions in Go provide organized, reusable code to perform a set of actions. Functions simplify the coding process, prevent redundant logic, and make code easier to follow. This topic describes the declaration and utilization of functions, arguments, parameters, return statements and scopes in Go.

Syntax#

func() // function type with no arguments and no return value
func(x int) int // accepts integer and returns an integer
func(a, b int, z float32) bool // accepts 2 integers, one float and returns a boolean
func(prefix string, values …int) // “variadic” function which accepts one string and one or more number of integers
func() (int, bool) // function returning two values
func(a, b int, z float64, opt …interface{}) (success bool) // accepts 2 integers, one float and one or more number of interfaces and returns named boolean value (which is already initialized inside of function)

Basic Declaration

A simple function that does not accept any parameters and does not return any values:

func SayHello() {
    fmt.Println("Hello!")
}

Parameters

A function can optionally declare a set of parameters:

func SayHelloToMe(firstName, lastName string, age int) {
    fmt.Printf("Hello, %s %s!\n", firstName, lastName)
    fmt.Printf("You are %d", age)
}

Notice that the type for firstName is omitted because it is identical to lastName.

Return Values

A function can return one or more values to the caller:

func AddAndMultiply(a, b int) (int, int) {
    return a+b, a*b
}

The second return value can also be the error var :

import errors

func Divide(dividend, divisor int) (int, error) {
    if divisor == 0 {
        return 0, errors.New("Division by zero forbidden")
    }
    return dividend / divisor, nil
}

Two important things must be noted:

The parenthesis may be omitted for a single return value.
Each return statement must provide a value for all declared return values.

Named Return Values

Return values can be assigned to a local variable. An empty return statement can then be used to return their current values. This is known as “naked” return. Naked return statements should be used only in short functions as they harm readability in longer functions:

func Inverse(v float32) (reciprocal float32) {
    if v == 0 {
        return
    }
    reciprocal = 1 / v
    return
}

play it on playground

//A function can also return multiple values
func split(sum int) (x, y int) {
    x = sum * 4 / 9
    y = sum - x
    return
}

play it on playground

Two important things must be noted:

The parenthesis around the return values are mandatory.
An empty return statement must always be provided.

Literal functions & closures

A simple literal function, printing Hello! to stdout:

package main

import "fmt"

func main() {
    func(){
        fmt.Println("Hello!")
    }()
}

play it on playground

A literal function, printing the str argument to stdout:

package main

import "fmt"

func main() {
    func(str string) {
        fmt.Println(str)
    }("Hello!")
}

play it on playground

A literal function, closing over the variable str:

package main

import "fmt"

func main() {
    str := "Hello!"
    func() {
        fmt.Println(str)
    }()
}

play it on playground

It is possible to assign a literal function to a variable:

package main

import (
    "fmt"
)

func main() {
    str := "Hello!"
    anon := func() {
        fmt.Println(str)
    }
    anon()
}

play it on playground

Variadic functions

A variadic function can be called with any number of trailing arguments. Those elements are stored in a slice.

package main

import "fmt"

func variadic(strs ...string) {
     // strs is a slice of string
     for i, str := range strs {
         fmt.Printf("%d: %s\n", i, str)
     }
}

func main() {
     variadic("Hello", "Goodbye")
     variadic("Str1", "Str2", "Str3")
}

play it on playground

You can also give a slice to a variadic function, with ...:

func main() {
     strs := []string {"Str1", "Str2", "Str3"}

     variadic(strs...)
}

play it on playground