69 lines
2.6 KiB
Markdown
69 lines
2.6 KiB
Markdown
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title = "Conversions with iota in go"
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date = 2020-04-17
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`iota` is a helpful way to enumerate constants in Go:
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```bash
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const (
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c0 = iota
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c1
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c2
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)
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func main() {
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fmt.Println(c0, c1, c2) // "0 1 2"
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}
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```
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But it is more flexible than just defining a constant to be an integer. According to the specification:
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> 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."
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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.
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However, although less seen, conversions can be pretty useful as well:
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```bash
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type Priority string
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const (
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_ = Priority("P" + string(iota+48))
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P1 // "P1"
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P2 // "P2"
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P3 // "P3"
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...
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)
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```
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It is not possible to use `strconv` functions in the `const` block because, well, they're functions. According to the specification:
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> "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."
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And that rule is:
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> "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"."
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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:
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```go
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type Grade string
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const (
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A = Grade(string(iota+65))
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B
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C
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)
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```
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## Sources
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- [golang.org](https://golang.org/ref/spec#Constants)
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- [golang.org](https://golang.org/doc/effective_go.html#constants)
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- [golang.org](https://golang.org/ref/spec#Conversions)
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- [golang.org](https://golang.org/ref/spec#Conversions_to_and_from_a_string_type)
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- [en.wikipedia.org](https://en.wikipedia.org/wiki/UTF-8)
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- [en.wikipedia.org](https://en.wikipedia.org/wiki/ASCII#Printable_characters)
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