Conversions with iota in go

17 Apr, 2020

iota is a helpful way to enumerate constants in Go:

const (
  c0 = iota
  c1
  c2
)

func main() {
  fmt.Println(c0, c1, c2) // "0 1 2"
}

But it is more flexible than just defining a constant to be an integer. According to the specification:

A constant value is represented by a rune, integer, floating-point, imaginary, or string literal, an identifier denoting a constant, a constant expression, a conversion with a result that is a constant, or the result value of some built-in functions such as unsafe.Sizeof applied to any value, cap or len applied to some expressions, real and imag applied to a complex constant and complex applied to numeric constants."

So we are allowed to use expressions and conversions on the right hand side of the = sign in a constant declaration. That means we can use iota in an expression or a conversion. A common use case for expression is creating a bitmask with the bit shift operator.

However, although less seen, conversions can be pretty useful as well:

type Priority string

const (
    _ = Priority("P" + string(iota+48))
    P1     // "P1"
    P2     // "P2"
    P3     // "P3"
    ...
)

It is not possible to use strconv functions in the const block because, well, they're functions. According to the specification:

"A constant value x can be converted to type T if x is representable by a value of T. As a special case, an integer constant x can be explicitly converted to a string type using the same rule as for non-constant x."

And that rule is:

"Converting a signed or unsigned integer value to a string type yields a string containing the UTF-8 representation of the integer. Values outside the range of valid Unicode code points are converted to "\uFFFD"."

The first 128 characters of UTF-8 are the same as the ASCII characters, prefixed with a 0. In ASCII the characters 0 to 9 are encoded sequentially from 48 until 57, so string(49) converts to "1". The letters of the alphabet are encoded sequentially as well, so a range of constants representing grades from A to F could be declared in the same fashion:

type Grade string

const (
  A = Grade(string(iota+65))
  B
  C
)

ewintr.nl

Conversions with iota in go

Sources