Go — Constants

Basic constants

package main

import "fmt"

const Pi = 3.14159
const MaxRetries = 3
const AppName = "MyApp"

func main() {
    fmt.Println(Pi)
    fmt.Println(MaxRetries)
    fmt.Println(AppName)
}

Typed constants

const (
    StatusOK    int = 200
    StatusError int = 500
    PI          float64 = 3.14159
    MaxSize     int64   = 1024
)

Untyped constants

const Big = 1 << 100        // Very large number
const Small = Big >> 99      // 2

// Untyped constants have higher precision
const Pi = 3.14159265358979323846

iota

Generate a sequence of constants automatically:

const (
    Sunday = iota    // 0
    Monday           // 1
    Tuesday          // 2
    Wednesday        // 3
    Thursday         // 4
    Friday           // 5
    Saturday         // 6
)

iota with expressions

const (
    _  = iota
    KB = 1 << (10 * iota) // 1024
    MB                    // 1048576
    GB                    // 1073741824
    TB                    // 1099511627776
)

const (
    FlagNone = 1 << iota // 1
    FlagRead             // 2
    FlagWrite            // 4
    FlagExecute          // 8
)

iota with custom start

const (
    A = iota + 10  // 10
    B              // 11
    C              // 12
)

Blank identifier with iota

const (
    _  = iota // skip 0
    A         // 1
    B         // 2
    C         // 3
)

Enum pattern

type Color int

const (
    Red   Color = iota // 0
    Green              // 1
    Blue               // 2
)

func (c Color) String() string {
    switch c {
    case Red:
        return "Red"
    case Green:
        return "Green"
    case Blue:
        return "Blue"
    default:
        return "Unknown"
    }
}

Constant block

const (
    Width  = 100
    Height = 200
    Area   = Width * Height // Can use other constants
)

Best practices

• Use const for values that never change
• Use iota for enumerations and bit flags
• Group related constants in a const block
• Use descriptive names for constants
• Avoid magic numbers — use named constants

Mini Practice

Write Go code that:

1. Declares constants for HTTP status codes
2. Uses iota to create a bit flag enum
3. Creates file size constants (KB, MB, GB)
4. Defines an enum type with a String() method

Up Next

In the next lesson, you'll learn about Data Types — the full set of types in Go.