Functions in Go

The literal of a function type is composed of the func keyword and a function signature literal. A function signature is composed of two type list, one is the input parameter type list, the other is the output result type lists. Parameter and result names can appear in function type and signature literals, but the names are not important.

In practice, the func keyword can be presented in signature literals, or not. For this reason, we can think function type and function signature as the same concept.

Here is a literal of a function type:

func (a int, b string, c string) (x int, y int, z bool)

From the article function declarations and calls, we have learned that consecutive parameters and results of the same type can be declared together. So the above literal is equivalent to

func (a int, b, c string) (x, y int, z bool)

As parameter names and result names are not important in the literals (as long as there are no duplicate non-blank names), the above ones are equivalent to the following one.

func (x int, y, z string) (a, b int, c bool)

Variable (parameter and result) names can be blank identifier _. The above ones are equivalent to the following one.

func (_ int, _, _ string) (_, _ int, _ bool)

The parameter names must be either all present or all absent (anonymous). The same rule is for result names. The above ones are equivalent to the following ones.

func (int, string, string) (int, int, bool) // the standard form
func (a int, b string, c string) (int, int, bool)
func (x int, _ string, z string) (int, int, bool)
func (int, string, string) (x int, y int, z bool)
func (int, string, string) (a int, b int, _ bool)

All of the above function type literals denote the same (unnamed) function type.

Each parameter list must be enclosed in a () in a literal, even if the parameter list is blank. If a result list of a function type is blank, then it can be omitted from literal of the function type. When a result list has most one result, then the result list doesn't need to be enclosed in a () if the literal of the result list contains no result names.

// The following three function types are identical.
func () (x int)
func () (int)
func () int

// The following two function types are identical.
func (a int, b string) ()
func (a int, b string)

The last parameter of a function can be a variadic parameter. Each function can have at most one variadic parameter. The type of a variadic parameter is always a slice type. To indicate the last parameter is variadic, just prefix three dots ... to the element type of its (slice) type in its declaration. Example:

func (values ...int64) (sum int64)
func (sep string, tokens ...string) string

A function type with variadic parameter can be called a variadic function type. A variadic function type and a non-variadic function type are absolutely not identical.

Some variadic functions examples will be shown in a below section.

A function prototype is composed of a function name and a function type (or signature). Its literal is composed of the func keyword, a function name and the literal of a function signature literal.

A function prototype literal example:

func Double(n int) (result int)

In other words, a function prototype is a function declaration without the body portion. A function declaration is composed of a function prototype and a function body.

Variadic function declarations are similar to general function declarations. The difference is that the last parameter of a variadic function must be variadic parameter. Note, the variadic parameter of a variadic function will be treated as a slice within the body of the variadic function.

// Sum and return the input numbers.
func Sum(values ...int64) (sum int64) {
	// The type of values is []int64.
	sum = 0
	for _, v := range values {
		sum += v
	}
	return
}

// An inefficient string concatenation function.
func Concat(sep string, tokens ...string) string {
	// The type of tokens is []string.
	r := ""
	for i, t := range tokens {
		if i != 0 {
			r += sep
		}
		r += t
	}
	return r
}

From the above two variadic function declarations, we can find that if a variadic parameter is declared with type portion as ...T, then the type of the parameter is []T actually.

In fact, the Print, Println and Printf functions in the fmt standard package are all variadic functions.

func Print(a ...interface{}) (n int, err error)
func Printf(format string, a ...interface{}) (n int, err error)
func Println(a ...interface{}) (n int, err error)

The variadic parameter types of the three functions are all []interface{}, which element type interface{} is an interface types. Interface types and values will be explained interfaces in Go later.

There are two manners to pass arguments to a variadic parameter of type []T:

1. pass a slice value as the only argument. The slice must be assignable to values of type []T, and the slice must be followed by three dots .... The passed slice is called as a variadic argument.
2. pass zero or more arguments which are assignable to values of type T. These arguments will be copied (or converted) as the elements of a new allocated slice value of type []T, then the new allocated slice will be passed to the variadic parameter.

Note, the two manners can't be mixed in the same variadic function call.

An example program which uses some variadic function calls:

package main

import "fmt"

func Sum(values ...int64) (sum int64) {
	sum = 0
	for _, v := range values {
		sum += v
	}
	return
}

func main() {
	a0 := Sum()
	a1 := Sum(2)
	a3 := Sum(2, 3, 5)
	// The above three lines are equivalent to
	// the following three respective lines.
	b0 := Sum([]int64{}...) // <=> Sum(nil...)
	b1 := Sum([]int64{2}...)
	b3 := Sum([]int64{2, 3, 5}...)
	fmt.Println(a0, a1, a3) // 0 2 10
	fmt.Println(b0, b1, b3) // 0 2 10
}

Another example:

package main

import "fmt"

func Concat(sep string, tokens ...string) (r string) {
	for i, t := range tokens {
		if i != 0 {
			r += sep
		}
		r += t
	}
	return
}

func main() {
	tokens := []string{"Go", "C", "Rust"}
	// manner 1
	langsA := Concat(",", tokens...)
	// manner 2
	langsB := Concat(",", "Go", "C","Rust")
	fmt.Println(langsA == langsB) // true
}

The following example doesn't compile, for the two variadic function call manners are mixed.

package main

// See above examples for the full declarations
// of the following two functions.
func Sum(values ...int64) (sum int64)
func Concat(sep string, tokens ...string) string

func main() {
	// The following two lines both fail
	// to compile, for the same error:
	// too many arguments in call.
	_ = Sum(2, []int64{3, 5}...)
	_ = Concat(",", "Go", []string{"C", "Rust"}...)
}

Generally, the names of the functions declared in the same code package can't be duplicate. But there are two exceptions.

1. One exception is each code package can declare several functions with the same name init and the same type func ().
2. The other exception is multiple functions can be declared with names as the blank identifier _, in which cases, the declared functions can never be called.

If a function has return results, then the last statement in its declaration body must be a terminating statement. Other than return terminating statement, there are some other kinds of terminating statements. So a function body is not required to contain a return statement. For example,

func fa() int {
	a:
	goto a
}

func fb() bool {
	for{}
}

A call to a function with single return result can always be used as a single value. For example, it can be nested in another function call as an argument, and can also be used as a single value to appear in any other expressions and statements.

If the return results of a call to a multi-result function are not discarded, then the call can only be used as a multi-value expression in two scenarios.

1. The call can be used in an assignment as source values. But the call can't mix with other source values in the assignment.
2. The call can be nested in another function call as arguments. But the call can't mix with other arguments.

An example:

package main

func HalfAndNegative(n int) (int, int) {
	return n/2, -n
}

func AddSub(a, b int) (int, int) {
	return a+b, a-b
}

func Dummy(values ...int) {}

func main() {
	// These lines compile okay.
	AddSub(HalfAndNegative(6))
	AddSub(AddSub(AddSub(7, 5)))
	AddSub(AddSub(HalfAndNegative(6)))
	Dummy(HalfAndNegative(6))
	_, _ = AddSub(7, 5)

	// The following lines fail to compile.
	/*
	_, _, _ = 6, AddSub(7, 5)
	Dummy(AddSub(7, 5), 9)
	Dummy(AddSub(7, 5), HalfAndNegative(6))
	*/
}

Note, for the standard Go compiler, some built-in functions break the universality of the just described rules above.

As mentioned above, function types are one kind of types in Go. A value of a function type is called a function value. The zero values of function types are represented with the predeclared nil.

When we declare a custom function, we also declared an immutable function value actually. The function value is identified by the function name. The type of the function value is represented as the literal by omitting the function name from the function prototype literal.

Note, built-in functions can't be used as values. init functions also can't be used as values.

Any function value can be invoked just like a declared function. It is fatal error to call a nil function to start a new goroutine. The fatal error is not recoverable and will make the whole program crash. For other situations, calls to nil function values will produce recoverable panics, including deferred function calls.

From the article value parts, we know that non-nil function values are multi-part values. After one function value is assigned to another, the two functions share the same underlying parts(s). In other words, the two functions represent the same internal function object. The effects of invoking two functions are the same.

An example:

package main

import "fmt"

func Double(n int) int {
	return n + n
}

func Apply(n int, f func(int) int) int {
	return f(n) // the type of f is "func(int) int"
}

func main() {
	fmt.Printf("%T\n", Double) // func(int) int
	// Double = nil // error: Double is immutable.

	var f func(n int) int // default value is nil.
	f = Double
	g := Apply // let compile deduce the type of g
	fmt.Printf("%T\n", g) // func(int, func(int) int) int

	fmt.Println(f(9))         // 18
	fmt.Println(g(6, Double)) // 12
	fmt.Println(Apply(6, f))  // 12
}

In the above example, g(6, Double) and Apply(6, f) are equivalent.

In practice, we often assign anonymous functions to function variables, so that we can call the anonymous functions multiple times.

package main

import "fmt"

func main() {
	// This function returns a function (a closure).
	isMultipleOfX := func (x int) func(int) bool {
		return func(n int) bool {
			return n%x == 0
		}
	}

	var isMultipleOf3 = isMultipleOfX(3)
	var isMultipleOf5 = isMultipleOfX(5)
	fmt.Println(isMultipleOf3(6))  // true
	fmt.Println(isMultipleOf3(8))  // false
	fmt.Println(isMultipleOf5(10)) // true
	fmt.Println(isMultipleOf5(12)) // false

	isMultipleOf15 := func(n int) bool {
		return isMultipleOf3(n) && isMultipleOf5(n)
	}
	fmt.Println(isMultipleOf15(32)) // false
	fmt.Println(isMultipleOf15(60)) // true
}

All functions in Go can be viewed as closures. This is why user experiences of all kinds of Go functions are so uniform and why Go functions are as flexible as those in dynamic languages.

• Go 101에 대해 - 이 책이 쓰여진 이유
• 감사의 말

• Go 소개 - Go를 배우는 가치
• Go 툴체인 - Go 프로그램을 컴파일하고 실행하는 방법

• Go 코드에 익숙해지기
  • 소스 코드 요소 소개
  • 키워드와 식별자
  • 기본 자료형과 기본 값 리터럴
  • 상수와 변수 - 무형성(untyped) 값과 자료형 추론 소개를 포함
  • 일반 연산자 - 더 많은 자료형 추론 규칙 소개를 포함
  • 함수 선언과 호출
  • 코드 패키지와 패키지 들여오기
  • 표현식, 구문과 단순 구문
  • 기본 흐름 제어
  • 고루틴, 지연된 함수 호출과 패닉/복구

• Go 자료형 체계
  • Go 자료형 체계 개요 - Go 프로그래밍 숙달을 위해 반드시 읽어봐야 하는
  • 포인터
  • 구조체
  • 변수 - Go 변수에 대한 더 깊은 이해
  • 배열, 슬라이스와 맵 - 1급 객체 컨테이너 자료형
  • 문자열
  • 함수 - 함수 자료형과 값, 가변 인자 함수
  • 채널 - Go에서 동시성 동기화를 하는 방법
  • 메서드
  • 인터페이스 - 리플렉션과 다형성을 하는 값 상자
  • 자료형 임베딩 - 자료형을 확장하는 방법
  • 자료형에 안전하지 않는 포인터
  • 제네릭 - 합성 자료형의 사용과 읽는 법
  • 리플렉션 - reflect 표준 패키지

• 특별 주제
  • 개행 규칙
  • 지연된 함수 호출 더 알아보기
  • 패닉/복구 사용 사례
  • 패닉/복구 메커니즘에 대한 고찰 - 함수 호출 종료 단계를 포함
  • 코드 블록과 식별자 스코프
  • 표현식 평가 순서
  • Go의 값 복사 비용
  • 경계 검사 제거(BCE)

• 동시성 프로그래밍
  • 동시성 동기화 개요
  • 채널 사용 사례
  • 채널을 깔끔하게 닫는 방법
  • 기타 동시성 동기화 기술 - sync 표준 패키지
  • 원자적 연산 - sync/atomic 표준 패키지
  • Go의 메모리 순서 보장
  • 흔히들 저지르는 동시성 프로그래밍 실수들

• 메모리 관련
  • 메모리 블록
  • 메모리 레이아웃
  • 메모리 누수 시나리오

• 일부 요약
  • 몇 가지 간단한 요약
  • Go의 nil
  • 값 변환, 할당과 비교 규칙
  • 구문/의미 예외
  • Go Details 101
  • Go FAQ 101
  • Go Tips 101

• 더 많은 토픽