Defining Constants in Go

In Go, constants are used to store fixed values that cannot be changed during the execution of a program. They are typically used for values that remain constant throughout the program, such as mathematical constants, configurations, or labels for specific states.

Introduction

A constant is a value that is defined once and cannot be modified by the program. Constants in Go can only be of the following types:

Boolean
Numeric (integer, float, or complex)
String

Explicit and Implicit Typing

In Go, constants are declared using the const keyword. Here’s the basic syntax:

const identifier [type] = value

identifier is the name of the constant.
type is optional and can be omitted if the type is implicitly clear.

Example without type:

const PI = 3.14159

In this case, the type is automatically inferred as float64.

Example with type:

const GREETING string = "hello"

It's also common to use uppercase names for constants to improve readability:

const INCHTOCM = 2.54

Typed and Untyped Constants

Go distinguishes between typed and untyped constants:

Typed constants: Declared with an explicit type.
Untyped constants: Type is inferred from the value and becomes fixed when used in a context that requires a specific type.

For example:

var n int
f(n + 5) // "5" becomes an int here because n is int

Compilation

Constants are evaluated at compile time, which means their values must be known before the program runs.

Valid example:

const C1 = 2 / 3 // OK, evaluated at compile time

Invalid example:

const C2 = getNumber() // Error: getNumber() cannot be evaluated at compile time

In the second example, getNumber() is a function that runs at runtime, so it cannot be used to assign a constant value.

Overflow

Go allows numeric constants to have arbitrary precision, meaning they won’t overflow.

Examples:

const Ln2 = 0.693147180559945309417232121458176568075500134360255254120680009
const Log2E = 1 / Ln2
const BILLION = 1e9 // float constant
const HARD_EIGHT = (1 << 100) >> 97

Here, Ln2 is declared across multiple lines using the backslash \, which allows for long constant declarations.

Multiple Assignments

Go allows declaring multiple constants in a single line. For example:

const BEEF, TWO, C = "meat", 2, "veg"

You can also declare multiple constants of the same type in one go:

const MONDAY, TUESDAY, WEDNESDAY, THURSDAY, FRIDAY, SATURDAY int = 1, 2, 3, 4, 5, 6

Here, all constants are explicitly typed as integers.

Enumerations

Constants can also be used to create enumerations, where a set of related values are grouped.

Example:

const (
  UNKNOWN = 0
  FEMALE  = 1
  MALE    = 2
)

The iota keyword can simplify this:

const (
  UNKNOWN = iota
  FEMALE
  MALE
)

The first iota is 0, and each subsequent constant gets the next integer value. This can also be typed:

type Gender int
const (
  UNKNOWN Gender = iota
  FEMALE
  MALE
)

Here, UNKNOWN, FEMALE, and MALE are constants of type Gender.

Full Code Example

Here’s a complete file with all the examples covered:

package main

import "fmt"

func main() {
    // Implicitly typed constants
    const PI = 3.14159
    const GREETING = "hello"
    fmt.Println(PI)      // 3.14159
    fmt.Println(GREETING) // hello

    // Explicitly typed constants
    const INCHTOCM float64 = 2.54
    const GREETING_EXPLICIT string = "hi"
    fmt.Println(INCHTOCM)           // 2.54
    fmt.Println(GREETING_EXPLICIT)   // hi

    // Typed and untyped constants
    var n int
    fmt.Println(n + 5) // 5 becomes int

    // Multiple assignments
    const BEEF, TWO, C = "meat", 2, "veg"
    fmt.Println(BEEF, TWO, C) // meat 2 veg

    const MONDAY, TUESDAY, WEDNESDAY int = 1, 2, 3
    fmt.Println(MONDAY, TUESDAY, WEDNESDAY) // 1 2 3

    // Enumeration with iota
    const (
        UNKNOWN = iota
        FEMALE
        MALE
    )
    fmt.Println(UNKNOWN, FEMALE, MALE) // 0 1 2

    // Typed enumeration with iota
    type Gender int
    const (
        UNKNOWN_GENDER Gender = iota
        FEMALE_GENDER
        MALE_GENDER
    )
    fmt.Println(UNKNOWN_GENDER, FEMALE_GENDER, MALE_GENDER) // 0 1 2

    // Overflow example
    const Ln2 = 0.693147180559945309417232121458176568075500134360255254120680009
    fmt.Println(Ln2)
}