{.epigraph}

I'm always delighted by the light touch and stillness of early programming languages. Not much text; a lot gets done. Old programs read like quiet conversations between a well-spoken research worker and a well- studied mechanical colleague, not as a debate with a compiler. Who'd have guessed sophistication bought such noise?

Quote: Richard P. Gabriel

Functions are the basic building blocks of Go programs; all interesting stuff happens in them.

Here is an example of how you can declare a function:

type mytype int
func (p mytype) funcname(q int) (r,s int) { return 0,0 }
//<<1>>        //<<2>>        //<<3>>      //<<4>>        //<<5>>         //<<6>>

To declare a function, you use the func keyword <<1>>. You can optionally bind <<2>> to a specific type called receiver (!functions, receiver) (a function with a receiver is usually called a method(!functions, method)). This will be explored in (#interfaces). Next <<3>> you write the name of your function. Here <<4>> we define that the variable q of type int is the input parameter. Parameters are passed pass-by-value.(!functions, pass-by-value) The variables r and s <<5>> are the named return parameters (((functions, named return parameters))) for this function. Functions in Go can have multiple return values. This is very useful to return a value and error. This removes the need for in-band error returns (such as -1 for EOF) and modifying an argument. If you want the return parameters not to be named you only give the types: (int, int). If you have only one value to return you may omit the parentheses. If your function is a subroutine and does not have anything to return you may omit this entirely. Finally, we have the body <<6>> of the function. Note that return is a statement so the braces around the parameter(s) are optional.

As said the return or result parameters of a Go function can be given names and used as regular variables, just like the incoming parameters. When named, they are initialized to the zero values for their types when the function begins. If the function executes a return statement with no arguments, the current values of the result parameters are returned. Using these features enables you (again) to do more with less code.^[This is a motto of Go; "Do more with less code".]

The names are not mandatory but they can make code shorter and clearer: they are documentation. However don't overuse this feature, especially in longer functions where it might not be immediately apparent what is returned.

Functions can be declared in any order you wish. The compiler scans the entire file before execution, so function prototyping is a thing of the past in Go. Go does not allow nested functions, but you can work around this with anonymous functions. See the Section (#functions-as-values) in this chapter. Recursive functions work just as in other languages:

func rec(i int) {
    if i == 10 { //<<1>>
        return
    }
    rec(i+1) //<<2>>
    fmt.Printf("%d ", i)
}

Here <<2>> we call the same function again, rec returns when i has the value 10, this is checked on the second line <<1>>. This function prints: 9 8 7 6 5 4 3 2 1 0, when called as rec(0).

Scope

Variables declared outside any functions are global (!scope, local) in Go, those defined in functions are local (!scope, local) to those functions. If names overlap - a local variable is declared with the same name as a global one

the local variable hides the global one when the current function is executed.

In the following example we call g() from f():

package main

var a int //<<1>>

func main() {
    a = 5
    print(a)
    f()
}

func f() {
    a := 6 //<<2>>
    print(a)
    g()
}

func g() {
    print(a)
}

Here <<1>>, we declare a to be a global variable of type int. Then in the main function we give the global a the value of 5, after printing it we call the function f. Then here <<2>>, a := 6, we create a new, local variable also called a. This new a gets the value of 6, which we then print. Then we call g, which uses the global a again and prints a's value set in main. Thus the output will be: 565. A local variable is only valid when we are executing the function in which it is defined. Note that the := used in line 12 is sometimes hard to spot so it is generally advised not to use the same name for global and local variables.

Functions as values

(!functions, as values) (!functions, literals) As with almost everything in Go, functions are also just values. They can be assigned to variables as follows:

<{{src/functions/anon-func.go}}[3,]

a is defined as an anonymous (nameless) function <<1>>. Note the lack of parentheses () after a. If there were, that would be to call some function with the name a before we have defined what a is. Once a is defined, then we can call it, <<3>>.

Functions--as--values may be used in other places, for example maps. Here we convert from integers to functions:

var xs = map[int]func() int{
    1: func() int { return 10 },
    2: func() int { return 20 },
    3: func() int { return 30 },
}

Note that the final comma on second to last line is mandatory.

Or you can write a function that takes a function as its parameter, for example a Map function that works on int slices. This is left as an exercise for the reader; see the exercise (#map-function).

Callbacks

Because functions are values they are easy to pass to functions, from where they can be used as callbacks. First define a function that does "something" with an integer value:

func printit(x int) {
    fmt.Printf("%v\n", x)
}

This function does not return a value and just prints its argument. The signature (!functions, signature) of this function is: func printit(int), or without the function name: func(int). To create a new function that uses this one as a callback we need to use this signature:

func callback(y int, f func(int)) {
    f(y)
}

Here we create a new function that takes two parameters: y int, i.e. just an int and f func(int), i.e. a function that takes an int and returns nothing. The parameter f is the variable holding that function. It can be used as any other function, and we execute the function on line 2 with the parameter y: f(y)

Deferred Code

Suppose you have a function in which you open a file and perform various writes and reads on it. In such a function there are often spots where you want to return early. If you do that, you will need to close the file descriptor you are working on. This often leads to the following code:

func ReadWrite() bool {
    file.Open("file")
    // Do your thing
    if failureX {
        file.Close() //<<1>>
        return false
    }

    if failureY {
        file.Close() //<<1>>
        return false
    }
    file.Close() //<<1>>
    return true  //<<2>>
}

Note that we repeat a lot of code here; you can see the that file.Close() is called at <<1>>. To overcome this, Go has the defer (!keywords, defer) keyword. After defer you specify a function which is called just before <<2>> the current function exits.

With defer we can rewrite the above code as follows. It makes the function more readable and it puts the Close right next to the Open.

func ReadWrite() bool {
    file.Open("filename")
    defer file.Close() //<<1>>
    // Do your thing
    if failureX {
        return false //<<2>>
    }
    if failureY {
        return false //<<2>>
    }
    return true //<<2>>
}

At <<1>> file.Close() is added to the defer list. (!keywords, defer list) Close is now done automatically at <<2>>. This makes the function shorter and more readable. It puts the Close right next to the Open.

You can put multiple functions on the "defer list", like this example from [@effective_go]:

for i := 0; i < 5; i++ {
    defer fmt.Printf("%d ", i)
}

Deferred functions are executed in LIFO order, so the above code prints: 4 3 2 1 0.

With defer you can even change return values, provided that you are using named result parameters and a function literal (!functions, literal)^[A function literal is sometimes called a closure (!closure).], i.e:

defer func() {/* ... */}()

Here we use a function without a name and specify the body of the function inline, basically we're creating a nameless function on the spot. The final braces are needed because defer needs a function call, not a function value. If our anonymous function would take an parameter it would be easier to see why we need the braces:

defer func(x int) {/* ... */}(5)

In this (unnamed) function you can access any named return parameter:

func f() (ret int)
    defer func() { //<<1>>
        ret++
    }()
    return 0
}

Here <<1>> we specify our function, the named return value ret is initialized with zero. The nameless function in the defer increments the value of ret with 1. The return 0 on line 5 will not be the returned value, because of defer. The function f will return 1!

Variadic Parameter

Functions that take a variable number of parameters are known as variadic functions. (!functions, variadic) To declare a function as variadic, do something like this:

func myfunc(arg ...int) {}

The arg ...int instructs Go to see this as a function that takes a variable number of arguments. Note that these arguments all have to have the type int. In the body of your function the variable arg is a slice of ints:

for _, n := range arg {
    fmt.Printf("And the number is: %d\n", n)
}

We range over the arguments on the first line. We are not interested in the index as returned by range, hence the use of the underscore there. In the body of the range we just print the parameters we were given.

If you don't specify the type of the variadic argument it defaults to the empty interface interface{} (see Chapter (#interfaces)).

Suppose we have another variadic function called myfunc2, the following example shows how to pass variadic arguments to it:

func myfunc(arg ...int) {
    myfunc2(arg...)
    myfunc2(arg[:2]...)
}

With myfunc2(arg...) we pass all the parameters to myfunc2, but because the variadic parameters is just a slice, we can use some slice tricks as well.

Panic and recovering

Go does not have an exception mechanism: you cannot throw exceptions. Instead it uses a panic-and-recover mechanism. It is worth remembering that you should use this as a last resort, your code will not look, or be, better if it is littered with panics. It's a powerful tool: use it wisely. So, how do you use it? In the words of the Go Authors [@go_blog_panic]:

Panic : is a built-in function that stops the ordinary flow of control and begins panicking. When the function F calls panic, execution of F stops, any deferred functions in F are executed normally, and then F returns to its caller. To the caller, F then behaves like a call to panic. The process continues up the stack until all functions in the current goroutine have returned, at which point the program crashes. Panics can be initiated by invoking panic directly. They can also be caused by runtime errors, such as out-of-bounds array accesses.

Recover : is a built-in function that regains control of a panicking goroutine. Recover is only useful inside deferred functions. During normal execution, a call to recover will return nil and have no other effect. If the current goroutine is panicking, a call to recover will capture the value given to panic and resume normal execution.

This function checks if the function it gets as argument will panic when it is executed^[Modified from a presentation of Eleanor McHugh.]:

func Panic(f func()) (b bool) { //<<1>>
    defer func() { //<<2>>
        if x := recover(); x != nil {
            b = true
        }
    }()
    f() //<<3>>
    return //<<4>>
}

We define a new function Panic <<1>> that takes a function as an argument (see (#functions-as-values)). It returns true if f panics when run, else false. We then <<2>> define a defer function that utilizes recover. If the current goroutine panics, this defer function will notice that. If recover() returns non-nil we set b to true. At <<3>> Execute the function we received as the argument. And finally <<4>> we return the value of b. Because b is a named return parameter.

The following code fragment, shows how we can use this function:

func panicy() {
    var a []int
    a[3] = 5
}

func main() {
    fmt.Println(Panic(panicy))
}

On line 3 the a[3] = 5 triggers a runtime out of bounds error which results in a panic. Thus this program will print true. If we change line 2: var a []int to var a [4]int the function panicy does not panic anymore. Why?

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