Go-Constant

A tiny, generic Go library for defining typed constant sets with safe string parsing.

Go has no built-in enum type. The common workaround — iota or typed strings — gives you no way to safely parse an unknown string into a valid constant. Go-Constant fills that gap with a single generic type that works with any value that implements fmt.Stringer.

Requirements

Go 1.18+ (generics)

Installation

go get github.com/sajjadrabiee/go-constant

Usage

String-based constants

type Direction string

func (d Direction) String() string { return string(d) }

const (
    North Direction = "north"
    South Direction = "south"
    East  Direction = "east"
    West  Direction = "west"
)

var Directions = constant.NewConstSet(North, North, South, East, West)

func main() {
    d, err := Directions.Parse("east")
    if err != nil {
        log.Fatal(err)
    }
    fmt.Println(d) // east

    _, err = Directions.Parse("up")
    fmt.Println(err) // "up" is not a valid constant
}

Struct-based constants

type Color struct {
    Name string
    Hex  string
}

func (c Color) String() string { return c.Name }

var (
    Red   = Color{Name: "red",   Hex: "#FF0000"}
    Green = Color{Name: "green", Hex: "#00FF00"}
    Blue  = Color{Name: "blue",  Hex: "#0000FF"}

    Colors = constant.NewConstSet(Red, Red, Green, Blue)
)

func main() {
    c, err := Colors.Parse("green")
    if err != nil {
        log.Fatal(err)
    }
    fmt.Println(c.Hex) // #00FF00
}

API

NewConstSet[V fmt.Stringer](defaultVal V, members ...V) ConstSet[V]

Creates a new constant set. The first argument is the default value returned when Parse fails.

(ConstSet[V]) Parse(value string) (V, error)

Converts a string to a member of the set by matching against each member's String() value. Returns the default value and a descriptive error if no match is found.

(ConstSet[V]) Members() []V

Returns all members of the set.

(ConstSet[V]) Default() V

Returns the default value of the set.

Why not iota?

Feature	iota	Go-Constant
Type-safe	Yes	Yes
String parsing	Manual switch	Built-in
Works with structs	No	Yes
List all values	Manual	Members()

Contributing

Contributions are welcome. Open an issue or submit a pull request.

License

MIT