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Function Declarations and Function Calls

Except the operator operations introduced in the last article, function operations are another kind of popular operations in programming. Function operations are often called function calls. This article will introduce how to declare functions and call functions.

Function Declarations

Let's view a function declaration.
func SquaresOfSumAndDiff(a int64, b int64) (s int64, d int64) {
	x, y := a + b, a - b
	s = x * x
	d = y * y
	return // <=> return s, d
We can find that, a function declaration is composed of several portions. From left to right,
  1. the first portion is the func keyword.
  2. the next portion is the function name, which must be an identifier. Here the function name is SquareOfSumAndDiff.
  3. the third portion is the input parameter declaration list, which is enclosed in a pair of ().
  4. the fourth portion is the output (or return) result declaration list. Go functions can return multiple results. For this specified example, the result declaration list is also enclosed in a pair of (). However, if the list is blank or it is composed of only one anonymous result declaration, then the pair of () in result declaration list is optional (see below for details).
  5. the last portion is the function body, which is enclosed in a pair of {}. In a function body, the return keyword is used to end the normal forward execution flow and enter the exiting phase (see the section after next) of a call of the function.

In the above example, each parameter and result declaration is composed of a name and a type (the type follows the name). We can view parameter and result declarations as standard variable declarations without the var keywords. The above declared function has two parameters, a and b, and has two results, s and d. All the types of the parameters and results are int64. Parameters and results are treated as local variables within their corresponding function bodies.

The names in the result declaration list of a function declaration can/must be present or absent all together. Either case is used common in practice. If a result is defined with a name, then the result is called a named result, otherwise, it is called an anonymous result.

When all the results in a function declaration are anonymous, then, within the corresponding function body, the return keyword must be followed by a sequence of return values, each of them corresponds to a result declaration of the function declaration. For example, the following function declaration is equivalent to the above one.
func SquaresOfSumAndDiff(a int64, b int64) (int64, int64) {
	return (a+b) * (a+b), (a-b) * (a-b)

In fact, if all the parameters are never used within the corresponding function body, the names in the parameter declaration list of a function declaration can be also be omitted all together. However, anonymous parameters are rarely used in practice.

Although it looks the parameter and result variables are declared outside of the body of a function declaration, they are viewed as general local variables within the function body. The difference is that local variables with non-blank names declared within a function body must be ever used in the function body. Non-blank names of top-level local variables, parameters and results in a function declaration can't be duplicated.

Go doesn't support default parameter values. The initial value of each result is the zero value of its type. For example, the following function will always print (and return) 0 false.
func f() (x int, y bool) {
	println(x, y) // 0 false

If the type portions of some successive parameters in a parameter declaration list are identical, then these parameters could share the same type portion in the parameter declaration list. The same is for result declaration lists. For example, the above two function declarations with the name SquaresOfSumAndDiff are equivalent to
func SquaresOfSumAndDiff(a, b int64) (s, d int64) {
	return (a+b) * (a+b), (a-b) * (a-b)
	// The above line is equivalent
	// to the following line.
	s = (a+b) * (a+b); d = (a-b) * (a-b); return

Please note, even if both the two results are named, the return keyword can be followed with return values.

If the result declaration list in a function declaration only contains one anonymous result declaration, then the result declaration list doesn't need to be enclosed in a (). If the function declaration has no return results, then the result declaration list portion can be omitted totally. The parameter declaration list portion can never be omitted, even if the number of parameters of the declared function is zero.

Here are more function declaration examples.
func CompareLower4bits(m, n uint32) (r bool) {
	r = m&0xF > n&0xF
	// The above two lines is equivalent to
	// the following line.
	return m&0xF > n&0xF

// This function has no parameters. The result
// declaration list is composed of only one
// anonymous result declaration, so it is not
// required to be enclosed within ().
func VersionString() string {
	return "go1.0"

// This function has no results. And all of
// its parameters are anonymous, for we don't
// care about them. Its result declaration
// list is blank (so omitted).
func doNothing(string, int) {

One fact we have learned from the earlier articles in Go 101 is that the main entry function in each Go program is declared without parameters and results.

Please note that, functions must be directly declared at package level. In other words, a function can't be declared within the body of another function. In a later section, we will learn that we can define anonymous functions in bodies of other functions. But anonymous functions are not function declarations.

Function Calls

A declared function can be called through its name plus an argument list. The argument list must be enclosed in a (). We often call this as argument passing (or parameter passing). Each single-value argument corresponds to (is passed to) a parameter.

Note: argument passing is also value assignments.

The type of an argument is not required to be identical with the corresponding parameter type. The only requirement for the argument is it must be assignable (a.k.a., implicitly convertible) to the corresponding parameter type.

If a function has return results, then each of its calls is viewed as an expression. If it returns multiple results, then each of its calls is viewed as a multi-value expression. A multi-value expression may be assigned to a list of destination values with the same count.

The following is a full example to show how to call some declared functions.
package main

func SquaresOfSumAndDiff(a int64, b int64) (int64, int64) {
	return (a+b) * (a+b), (a-b) * (a-b)

func CompareLower4bits(m, n uint32) (r bool) {
	r = m&0xF > n&0xF

// Initialize a package-level variable
// with a function call.
var v = VersionString()

func main() {
	println(v) // v1.0
	x, y := SquaresOfSumAndDiff(3, 6)
	println(x, y) // 81 9
	b := CompareLower4bits(uint32(x), uint32(y))
	println(b) // false
	// "Go" is deduced as a string,
	// and 1 is deduced as an int32.
	doNothing("Go", 1)

func VersionString() string {
	return "v1.0"

func doNothing(string, int32) {

From the above example, we can learn that a function can be either declared before or after any of its calls.

Function calls can be deferred or invoked in new goroutines (green threads) in Go. Please read a later article for details.

Exiting (or Returning) Phase of a Function Call

In Go, besides the normal forward execution phase, a function call may undergo an exiting phase (also called returning phase). The exiting phase of a function call starts when the called function is returned. In other words, when a function call is returned, it is possible that it hasn't exited yet.

More detailed explanations for exiting phases of function calls can be found in this article.

Anonymous Functions

Go supports anonymous functions. The definition of an anonymous function is almost the same as a function declaration, except there is no function name portion in the anonymous function definition.

An anonymous function can be called right after it is defined. Example:
package main

func main() {
	// This anonymous function has no parameters
	// but has two results.
	x, y := func() (int, int) {
		println("This function has no parameters.")
		return 3, 4
	}() // Call it. No arguments are needed.

	// The following anonymous function have no results.

	func(a, b int) {
		// The following line prints: a*a + b*b = 25
		println("a*a + b*b =", a*a + b*b)
	}(x, y) // pass argument x and y to parameter a and b.

	func(x int) {
		// The parameter x shadows the outer x.
		// The following line prints: x*x + y*y = 32
		println("x*x + y*y =", x*x + y*y)
	}(y) // pass argument y to parameter x.

	func() {
		// The following line prints: x*x + y*y = 25
		println("x*x + y*y =", x*x + y*y)
	}() // no arguments are needed.

Please note that, the last anonymous function is in the scope of the x and y variables declared above, it can use the two variables directly. Such functions are called closures. In fact, all custom functions in Go can be viewed as closures. This is why Go functions are as flexible as many dynamic languages.

Later, we will learn that an anonymous function can be assigned to a function value and can be called at any time later.

Built-in Functions

There are some built-in functions in Go, for example, the println and print functions. We can call these functions without importing any packages.

We can use the built-in real and imag functions to get the real and imaginary parts of a complex value. We can use the built-in complex function to produce a complex value. Please note, if any of the arguments of a call to any of the two functions are all constants, then the call will be evaluated at compile time, and the result value of the call is also a constant. In particular, if any of the arguments is an untyped constant, then the result value is also an untyped constant. The call is viewed as a constant expression.

// c is a untyped complex constant.
const c = complex(1.6, 3.3)

// The results of real(c) and imag(c) are both
// untyped floating-point values. They are both
// deduced as values of type float32 below.
var a, b float32 = real(c), imag(c)

// d is deduced as a typed value of type complex64.
// The results of real(d) and imag(d) are both
// typed values of type float32.
var d = complex(a, b)

// e is deduced as a typed value of type complex128.
// The results of real(e) and imag(e) are both
// typed values of type float64.
var e = c

More built-in functions will be introduced in other Go 101 articles later.

More About Functions

There are more about function related concepts and details which are not touched in the current article. We can learn those concepts and details in the article function types and values later.


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