Iota

With Go iota we gain a way to generate groups of constants based on expressions. Within a const block, iota always starts at 0 and is incremented by 1 for each item within the block.

We cannot use iota in an expression that must be evaluated at runtime. The term iota is like beta: it stands for the letter "I" in Greek, just as beta stands for B.

This Go example uses iota in several const blocks within a program. It shows how iota behaves when we do not use it for an item within a const.

Part 1 Iota is used within a const block with multiple items. It starts at 0 (for the value one) and continues to 1 and 2.

Part 2 If a const entry does not use iota, it is still incremented for that entry. It does not stop incrementing even if unused.

Part 3 Iota can be referenced any number of times within a const entry, but it is only incremented on the following const entry.

Part 4 We print out all the uses of iota in the program. Iota is like a counter for const lines within a block.

package main

import "fmt"

// Part 1: iota goes from 0 to 2 here.
const (
    one = iota
    two
    three
)

// Part 2: iota starts at 0 for the first const, and is incremented for each following value.
const (
    a = 10   // Iota is 0 (but not used)
    b = iota // Iota is 1
    c = 20   // Iota is 2 (not used)
    d = iota // Iota is 3
)

// Part 3: iota is the same for each single const value.
const (
    north = iota * iota // Iota is 0
    east  = iota * iota // Iota is 1
    south = iota * iota // Iota is 2
    west  = iota * iota // Iota is 3
)

func main() {
    // Part 4: print out the constants.
    fmt.Println(one, two, three)
    fmt.Println(a, b, c, d)
    fmt.Println(north, east, south, west)
}0 1 2
10 1 20 3
0 1 4 9

Iota cannot be initialized to start at a different value. And it cannot be told to skip over an entry in a const block. If used on a single const, it always just means 0.

With const and iota, the Go language provides compile-time code generation features. We can specify many constants with just an expression.