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    Go by Example: Enums

    Go by Example : Enums

    Enumerated types (enums) are a special case of sum types . An enum is a type that has a fixed number of possible values, each with a distinct name. Go doesn’t have an enum type as a distinct language feature, but enums are simple to implement using existing language idioms.

    package main
    import "fmt"

    Our enum type ServerState has an underlying int type.

    type ServerState int

    The possible values for ServerState are defined as constants. The special keyword iota generates successive constant values automatically; in this case 0, 1, 2 and so on.

    const (
    StateIdle ServerState = iota
    StateConnected
    StateError
    StateRetrying
)

    By implementing the fmt.Stringer interface, values of ServerState can be printed out or converted to strings.

    This can get cumbersome if there are many possible values. In such cases the stringer tool can be used in conjunction with go:generate to automate the process. See this post for a longer explanation.

    var stateName = map[ServerState]string{
    StateIdle:      "idle",
    StateConnected: "connected",
    StateError:     "error",
    StateRetrying:  "retrying",
}
    func (ss ServerState) String() string {
    return stateName[ss]
}

    If we have a value of type int , we cannot pass it to transition - the compiler will complain about type mismatch. This provides some degree of compile-time type safety for enums.

    func main() {
    ns := transition(StateIdle)
    fmt.Println(ns)
        ns2 := transition(ns)
    fmt.Println(ns2)
}

    transition emulates a state transition for a server; it takes the existing state and returns a new state.

    func transition(s ServerState) ServerState {
    switch s {
    case StateIdle:
        return StateConnected
    case StateConnected, StateRetrying:

    Suppose we check some predicates here to determine the next state…

            return StateIdle
    case StateError:
        return StateError
    default:
        panic(fmt.Errorf("unknown state: %s", s))
    }
}
    $ go run enums.go
connected
idle

    Next example: Struct Embedding .

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