an example of a promises in go, though i think i wrote this while following a tutorial, i cant remember tbh

promises-example.go

package main

import (
    "errors"
    "fmt"
    "time"
)

func main() {
    po := new(PurchaseOrder)
    po.Value = 4.20

    SavePO(po, false).Then(func(obj interface{}) error {
        po := obj.(*PurchaseOrder)
        fmt.Printf("Purchase Order Saved: %d\n", po.Number)
        return nil
    }, func(err error) {
        fmt.Printf("Failed to save purchase order:" + err.Error() + "\n")
    }).Then(func(obj interface{}) error {
        fmt.Println("Second Promise success")
        return nil
    }, func(err error) {
        fmt.Println("Second promise fail")
    })

    fmt.Scanln()
}

type PurchaseOrder struct {
    Number int
    Value  float64
}

func SavePO(po *PurchaseOrder, shouldFail bool) *Promise {
    result := new(Promise)
    result.successChannel = make(chan interface{}, 1)
    result.failureChannel = make(chan error, 1)

    go func() {
        time.Sleep(2 * time.Second)
        if shouldFail {
            result.failureChannel <- errors.New("Failed to save")
        } else {
            po.Number = 1234
            result.successChannel <- po
        }
    }()
    return result
}

type Promise struct {
    successChannel chan interface{}
    failureChannel chan error
}

func (this *Promise) Then(success func(interface{}) error, failure func(error)) *Promise {
    result := new(Promise)
    result.successChannel = make(chan interface{}, 1)
    result.failureChannel = make(chan error, 1)

    timeout := time.After(1 * time.Second)

    go func() {
        select {
        case obj := <-this.successChannel:
            newErr := success(obj)
            if newErr == nil {
                result.successChannel <- obj
            } else {
                result.failureChannel <- newErr
            }
        case err := <-this.failureChannel:
            failure(err)
            result.failureChannel <- err
        case <-timeout:
            failure(errors.New("Timed Out"))
        }
    }()
    return result
}