Promises library for Golang. Inspired by JS Promises.
Supports:
• Automatic panic recovery
• Nested promise flattening
• Promise cancellation
Note that the library will be updated as soon as generics arrive in Go 2.0 to allow for greater type safety, less boilerplate and better performance
$ go get -u github.com/chebyrash/promise
var p = promise.New(func(resolve func(interface{}), reject func(error)) {
// Do something asynchronously.
const sum = 2 + 2
// If your work was successful call resolve() passing the result.
if sum == 4 {
resolve(sum)
return
}
// If you encountered an error call reject() passing the error.
if sum != 4 {
reject(errors.New("2 + 2 doesnt't equal 4"))
return
}
// If you forgot to check for errors and your function panics the promise will
// automatically reject.
// panic() == reject()
})
// You may continue working with the result of
// a previous async operation. Calling .Then returns a new promise.
var resultP = p.Then(func(data interface{}) interface{} {
fmt.Println("The result is:", data)
return data.(int) + 1
})
// Callbacks can be added even after the success or failure of the asynchronous operation.
// Multiple callbacks may be added by calling .Then or .Catch several times,
// to be executed independently in insertion order.
// Each call returns a new promise, allowing for branching chains of promises.
var caughtP = p.
Then(func(data interface{}) interface{} {
fmt.Println("The new result is:", data)
return nil
}).
Catch(func(error error) error {
fmt.Println("Error during execution:", error.Error())
return nil
})
// Since callbacks are executed asynchronously you can wait for them.
// Wait for the result
resultP.Await()
// Wait for the caught branch
caughtP.Await()All methods except .Await() return new promises allowing for branching logic.
Wait for all promises to be resolved, or for any to be rejected. If the returned promise resolves, it is resolved with an aggregating array of the values from the resolved promises in the same order as defined in the iterable of multiple promises. If it rejects, it is rejected with the reason from the first promise in the iterable that was rejected.
Example:
var p1 = promise.Resolve(123)
var p2 = promise.Resolve("Hello, World")
var p3 = promise.Resolve([]string{"one", "two", "three"})
results, _ := promise.All(p1, p2, p3).Await()
fmt.Println(results)
// [123 Hello, World [one two three]]Wait until all promises have settled (each may resolve, or reject). Returns a promise that resolves after all of the given promises have either resolved or rejected, with an array of objects that each describe the outcome of each promise.
Example:
var p1 = promise.Resolve(123)
var p2 = promise.Reject(errors.New("something wrong"))
results, _ := promise.AllSettled(p1, p2).Await()
for _, result := range results.([]interface{}) {
switch value := result.(type) {
case error:
fmt.Printf("Bad error occurred: %s", value.Error())
default:
fmt.Printf("Other result type: %d", value.(int))
}
}
// Other result type: 123
// Bad error occurred: something wrongWait until any of the promises is resolved or rejected. If the returned promise resolves, it is resolved with the value of the first promise in the iterable that resolved. If it rejects, it is rejected with the reason from the first promise that was rejected.
Example:
var p1 = promise.Resolve("Promise 1")
var p2 = promise.Resolve("Promise 2")
fastestResult, _ := promise.Race(p1, p2).Await()
fmt.Printf("Both resolve, but %s is faster", fastestResult)
// Both resolve, but Promise 1 is faster
// OR
// Both resolve, but Promise 2 is fasterReturns a new Promise that is resolved with the given value. If the value is a thenable (i.e. has a then method), the returned promise will "follow" that thenable, adopting its eventual state; otherwise the returned promise will be fulfilled with the value.
Example:
var p1 = promise.Resolve("Hello, World")
result, _ := p1.Await()
fmt.Println(result)
// Hello, WorldReturns a new Promise that is rejected with the given reason.
Example:
var p1 = promise.Reject(errors.New("bad error"))
_, err := p1.Await()
fmt.Println(err)
// bad errorfunc parseJSON(data []byte) *promise.Promise {
return promise.New(func(resolve func(interface{}), reject func(error)) {
var body = make(map[string]string)
err := json.Unmarshal(data, &body)
if err != nil {
reject(err)
return
}
resolve(body)
})
}
func main() {
var requestPromise = promise.New(func(resolve func(interface{}), reject func(error)) {
var url = "https://httpbin.org/ip"
resp, err := http.Get(url)
defer resp.Body.Close()
if err != nil {
reject(err)
return
}
body, err := ioutil.ReadAll(resp.Body)
if err != nil {
reject(err)
return
}
resolve(body)
})
requestPromise.
// Parse JSON body
Then(func(data interface{}) interface{} {
// This can be a promise, it will automatically flatten
return parseJSON(data.([]byte))
}).
// Work with parsed body
Then(func(data interface{}) interface{} {
var body = data.(map[string]string)
fmt.Println("[Inside Promise] Origin:", body["origin"])
return body
}).
Catch(func(error error) error {
fmt.Println(error.Error())
return nil
})
// Your resolved values can be extracted from the Promise
// But you are encouraged to handle them in .Then and .Catch
value, err := requestPromise.Await()
if err != nil {
fmt.Println("Error: " + err.Error())
return
}
origin := value.(map[string]string)["origin"]
fmt.Println("[Outside Promise] Origin: " + origin)
}func findFactorial(n int) int {
if n == 1 {
return 1
}
return n * findFactorial(n-1)
}
func findFactorialPromise(n int) *promise.Promise {
return promise.Resolve(findFactorial(n))
}
func main() {
var factorial1 = findFactorialPromise(5)
var factorial2 = findFactorialPromise(10)
var factorial3 = findFactorialPromise(15)
// Results calculated asynchronously
results, _ := promise.All(factorial1, factorial2, factorial3).Await()
values := results.([]interface{})
fmt.Println("Result of 5! is", values[0])
fmt.Println("Result of 10! is", values[1])
fmt.Println("Result of 15! is", values[2])
}var p = promise.Resolve(0)
p.
Then(func(data interface{}) interface{} {
fmt.Println("I will execute first")
return nil
}).
Then(func(data interface{}) interface{} {
fmt.Println("And I will execute second!")
return nil
}).
Then(func(data interface{}) interface{} {
fmt.Println("Oh I'm last :(")
return nil
})
p.Await()