How do constants and iota work in Go?

Constants are declared with const and must be assigned a value that is computable at compile time — no function calls or runtime values. The iota identifier provides an automatically incrementing integer within a const block, resetting to 0 at the start of each new block.

// Simple constants
const Pi    = 3.14159
const AppName = "MyService"

// iota: auto-incrementing integer, resets at each const block
type Weekday int
const (
    Sunday Weekday = iota  // 0
    Monday                 // 1
    Tuesday                // 2
    Wednesday              // 3
    Thursday               // 4
    Friday                 // 5
    Saturday               // 6
)

// iota with bit-shifting — perfect for flag constants
type Permission uint
const (
    Read    Permission = 1 << iota  // 1 (001)
    Write                           // 2 (010)
    Execute                         // 4 (100)
)
userPerms := Read | Write  // 3 — can read and write

// iota with expressions
const (
    _  = iota              // skip 0
    KB = 1 << (10 * iota)  // 1 << 10 = 1024
    MB                     // 1 << 20
    GB                     // 1 << 30
)

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