Getting a promise's value via yield & co - javascript

I'm trying to figure out how to get the value of a promise via yield, possibly with "co":
function *(){
var someVar = yield functionThatReturnsAPromise();
}
The called function is not a generator, just a normal function. With the above, someVar == Promise, but I want the resolved value. Does co or some other library have a way of doing this?

Typically a yield acts returns the same value to its own paused execution (left hand side of the yield function) as to the calling function of the generator. In this simple example counting from 1 to 5 example the input of the yield is the output of the yield to the generator function as well as to the generator's execution path:
function* inc() {
var g = 0;
while (true) {
g = yield g + 1;
}
}
var incGen = inc();
for (i = incGen.next().value; i <= 5; i = incGen.next(i).value) {
console.log(i); // ^ input of generator is last output
}
However, the calling function may also call the generator, but replace the output the last yield with another value or even throw an exception to the generator's execution. In the case of promise a function that returns a promise, may yield the result of that promise instead of the promise itself. So in this case:
var someVar = yield functionThatReturnsAPromise();
^ output != ^ input
you want the yield to act as a function that takes a promise as an input and returns a resolved promise as an output to the generator function.
It so happens co is able to do exactly this for you. All you need to do is feed your generator function to the co function:
co(function *(){
var someVar = yield functionThatReturnsAPromise();
})
To better understand how this works, here is an example of a function, that does the same thing as co:
function async(makeGenerator){
return function (){
var generator = makeGenerator.apply(this, arguments)
function handle(result){
if (result.done) return result.value
return result.value.then(function (res){
return handle(generator.next(res)) // <- sets output of yield to the promise result
}, function (err){ // and returns input promise
return handle(generator.throw(err)) // <- throw promise exception to generator function
})
}
return handle(generator.next()) // <- first time call, input of yield is a promise
}
}
source is from Forbes Lindesay's now famous presentation about this concept

Yes, co (https://github.com/tj/co) can do that. You'll have to wrap parent function inside co call:
co(function *(){
var someVar = yield functionThatReturnsAPromise();
})()
someVar inside will become resolved value. If promise gets rejected, error can be cought with basic try {} catch (e) {} statements.

Related

How will coroutine behaves when the promise is resolved multiple times and co-routine have next yield

For promise & co-routine, I've used bluebird npm package in NodeJs.
Can someone please help in understanding the behavior of below code when promise is getting resolved multiple times.
Question:
What will happens to coroutine when the promise resolved for multiple times?
Will the second yield will be affected by the multiple yield of first.
const bluebird = require("bluebird");
function func1() {
return new bluebird((resolve, reject) => {
let c = 0;
let iterval = SetInterval(() => {
c++;
let cc = c;
console.log(`c=${c}`);
if(cc === 20) {
clearInterval(interval);
}
resolve(true);
}, 1000);
});
}
let run1 = bluebird.coroutine(function*() {
try {
yield func1();
yield func1();
yield func1();
yield func1();
} catch (e) {
console.dir(e);
}
});
Nothing. resolve and reject are issued as a pair of anonymous functions by the promise constructor. Once one of them has been called, further call to either of them are ignored.
Note new interval timers are called each time func1 is called, without stopping those previously started, so expect the console output to look messy.
Also each call to func1 returns a different, new promise.
No because the postulate is incorrect. The first yield does not yield multiple times, it returns a promise that is resolved after one second. It's in a generator function, which resumes at the line after the previous yield (the last time I checked). The coroutine is about calling the generator function after the promise returned in the previous yield becomes settled. See http://bluebirdjs.com/docs/api/promise.coroutine.html for more details.

Generator function with name

I have just started using Bluebird's Promise.coroutine which is a promise version of Generator functions from ES6.
Everything works fine when I create a function and put it in a variable. Like:
let success = Promise.coroutine(function* (_context) {
...
});
exports.success = Promise.coroutine(function* (_context) {
...
});
But when I try to create an standalone function. Like:
Promise.coroutine(function *success() {
...
});
It never defines a function and I get the error:
success is not defined
How do I access an standalone generator function? or more straight forward, how to create it?
Edit:
I am using validatejs, it requires success and error functions for async validations:
exports.create = function (req, res) {
var constraints = {
...
}
validate.async(req, constraints).then(Promise.coroutine(success), Promise.coroutine(error));
function success() { //generator
}
function error(e) { //generator
}
}
You can define a generator function as shown below.
function* functionName([param[, param[, ... param]]]) {
statements..
}
Please note that symbol * is with word function and not the functionname. The declaration function keyword followed by an asterisk defines a generator function.
Update1: Usage with the Promise.coroutine method. In javascript, function are first class citizen and hence can be passed as an parameter. So, you can replace the function expression with the functionname.
Promise.coroutine(functionName);
Your success() function doesn't have to be named because you are actualyy not calling that, but calling the coroutine Promise. See the example below. You should assign your coroutine to whatever you are trying to call it from, and then yield a Promise for your delayed processing (whatever that may be). Then you need to call the coroutine that will take care of returning the promise.
var Promise = require("bluebird");
function Test() {
}
Test.prototype.foo = Promise.coroutine(function* success() {
console.log("Called success")
var i = 0;
while (i < 3) {
console.log("Waiting and Yield " + i++);
yield Promise.delay(1000);
}
console.log("Test " + i);
});
var a = new Test();
a.foo();
console.log("Done!");
Then you will get this output :
>node index.js
Called success
Waiting and Yield 0
Done!
Waiting and Yield 1
Waiting and Yield 2
Test 3

Difference between async/await and ES6 yield with generators

I was just reading this fantastic article «Generators» and it clearly highlights this function, which is a helper function for handling generator functions:
function async(makeGenerator){
return function () {
var generator = makeGenerator.apply(this, arguments);
function handle(result){
// result => { done: [Boolean], value: [Object] }
if (result.done) return Promise.resolve(result.value);
return Promise.resolve(result.value).then(function (res){
return handle(generator.next(res));
}, function (err){
return handle(generator.throw(err));
});
}
try {
return handle(generator.next());
} catch (ex) {
return Promise.reject(ex);
}
}
}
which I hypothesize is more or less the way the async keyword is implemented with async/await. So the question is, if that is the case, then what the heck is the difference between the await keyword and the yield keyword? Does await always turn something into a promise, whereas yield makes no such guarantee? That is my best guess!
You can also see how async/await is similar to yield with generators in this article where he describes the 'spawn' function ES7 async functions.
Well, it turns out that there is a very close relationship between async/await and generators. And I believe async/await will always be built on generators. If you look at the way Babel transpiles async/await:
Babel takes this:
this.it('is a test', async function () {
const foo = await 3;
const bar = await new Promise(resolve => resolve('7'));
const baz = bar * foo;
console.log(baz);
});
and turns it into this
function _asyncToGenerator(fn) {
return function () {
var gen = fn.apply(this, arguments);
return new Promise(function (resolve, reject) {
function step(key, arg) {
try {
var info = gen[key](arg);
var value = info.value;
} catch (error) {
reject(error);
return;
}
if (info.done) {
resolve(value);
} else {
return Promise.resolve(value).then(function (value) {
return step("next", value);
}, function (err) {
return step("throw", err);
});
}
}
return step("next");
});
};
}
this.it('is a test', _asyncToGenerator(function* () { // << now it's a generator
const foo = yield 3; // <<< now it's yield, not await
const bar = yield new Promise(resolve => resolve(7));
const baz = bar * foo;
console.log(baz);
}));
you do the math.
This makes it look like the async keyword is just that wrapper function, but if that's the case then await just gets turned into yield, there will probably be a bit more to the picture later on when they become native.
You can see more of an explanation for this here:
https://www.promisejs.org/generators/
yield can be considered to be the building block of await. yield takes the value it's given and passes it to the caller. The caller can then do whatever it wishes with that value (1). Later the caller may give a value back to the generator (via generator.next()) which becomes the result of the yield expression (2), or an error that will appear to be thrown by the yield expression (3).
async-await can be considered to use yield. At (1) the caller (i.e. the async-await driver - similar to the function you posted) will wrap the value in a promise using a similar algorithm to new Promise(r => r(value) (note, not Promise.resolve, but that's not a big deal). It then waits for the promise to resolve. If it fulfills, it passes the fulfilled value back at (2). If it rejects, it throws the rejection reason as an error at (3).
So the utility of async-await is this machinery that uses yield to unwrap the yielded value as a promise and pass its resolved value back, repeating until the function returns its final value.
what the heck is the difference between the await keyword and the yield keyword?
The await keyword is only to be used in async functions, while the yield keyword is only to be used in generator function*s. And those are obviously different as well - the one returns promises, the other returns generators.
Does await always turn something into a promise, whereas yield makes no such guarantee?
Yes, await will call Promise.resolve on the awaited value.
yield just yields the value outside of the generator.
tl;dr
Use async/await 99% of the time over generators. Why?
async/await directly replaces the most common workflow of promise chains allowing code to be declared as if it was synchronous, dramatically simplifying it.
Generators abstract the use case where you would call a series of async-operations that depend on each other and eventually will be in a "done" state. The most simple example would be paging through results that eventually return the last set but you would only call a page as needed, not immediately in succession.
async/await is actually an abstraction built on top of generators to make working with promises easier.
See very in-depth Explanation of Async/Await vs. Generators
Try this test programs which I used to understand await/async with promises.
Program #1: without promises it doesn't run in sequence
function functionA() {
console.log('functionA called');
setTimeout(function() {
console.log('functionA timeout called');
return 10;
}, 15000);
}
function functionB(valueA) {
console.log('functionB called');
setTimeout(function() {
console.log('functionB timeout called = ' + valueA);
return 20 + valueA;
}, 10000);
}
function functionC(valueA, valueB) {
console.log('functionC called');
setTimeout(function() {
console.log('functionC timeout called = ' + valueA);
return valueA + valueB;
}, 10000);
}
async function executeAsyncTask() {
const valueA = await functionA();
const valueB = await functionB(valueA);
return functionC(valueA, valueB);
}
console.log('program started');
executeAsyncTask().then(function(response) {
console.log('response called = ' + response);
});
console.log('program ended');
Program #2: with promises
function functionA() {
return new Promise((resolve, reject) => {
console.log('functionA called');
setTimeout(function() {
console.log('functionA timeout called');
// return 10;
return resolve(10);
}, 15000);
});
}
function functionB(valueA) {
return new Promise((resolve, reject) => {
console.log('functionB called');
setTimeout(function() {
console.log('functionB timeout called = ' + valueA);
return resolve(20 + valueA);
}, 10000);
});
}
function functionC(valueA, valueB) {
return new Promise((resolve, reject) => {
console.log('functionC called');
setTimeout(function() {
console.log('functionC timeout called = ' + valueA);
return resolve(valueA + valueB);
}, 10000);
});
}
async function executeAsyncTask() {
const valueA = await functionA();
const valueB = await functionB(valueA);
return functionC(valueA, valueB);
}
console.log('program started');
executeAsyncTask().then(function(response) {
console.log('response called = ' + response);
});
console.log('program ended');
In many ways, generators are a superset of async/await. Right now async/await has cleaner stack traces than co, the most popular async/await-like generator based lib. You can implement your own flavor of async/await using generators and add new features, like built-in support for yield on non-promises or building it on RxJS observables.
So, in short, generators give you more flexibility and generator-based libs generally have more features. But async/await is a core part of the language, it's standardized and won't change under you, and you don't need a library to use it. I have a blog post with more details on the difference between async/await and generators.
The yield+gen.next()-as-a-language-feature can be used to describe (or implement) the underlying control-flow that await-async has abstracted away.
As other answers suggest, await-as-a-language-feature is (or can be thought of) an implementation on top of yield.
Here is a more intutive understanding for that:
Say we have 42 awaits in an async function, await A -> await B -> ...
Deep down it is equivalent to having yield A -> tries resolve this as a Promise [1]
-> if resolvable, we yield B, and repeat [1] for B
-> if not resolveable, we throw
And so we end up with 42 yields in a generator. And in our controller we simply keep doing gen.next() until it is completed or gets rejected. (ie this is the same as using await on an async function that contains 42 await.)
This is why lib like redux-saga utilizes generator to then pipe the promises to the saga middleware to be resolved all at one place; thus decoupling the Promises constructions from their evaluations, thus sharing close resemblance to the Free Monad.
The idea is to recursively chain then() calls to replicate the behavior of await which allows one to invoke async routines in a synchronous fashion. A generator function is used to yield back control (and each value) from the callee to the caller, which happens to be the _asyncToGenerator() wrapper function.
As mentioned above, this is the trick that Babel uses to create polyfills. I slightly edited the code to make it more readable and added comments.
(async function () {
const foo = await 3;
const bar = await new Promise((resolve) => resolve(7));
const baz = bar * foo;
console.log(baz);
})();
function _asyncToGenerator(fn) {
return function () {
let gen = fn(); // Start the execution of the generator function and store the generator object.
return new Promise(function (resolve, reject) {
function step(func, arg) {
try {
let item = gen[func](arg); // Retrieve the function object from the property name and invoke it. Similar to eval(`gen.${func}(arg)`) but safer. If the next() method is called on the generator object, the item value by the generator function is saved and the generator resumes execution. The value passed as an argument is assigned as a result of a yield expression.
if (item.done) {
resolve(item.value);
return; // The executor return value is ignored, but we need to stop the recursion here.
}
// The trick is that Promise.resolve() returns a promise object that is resolved with the value given as an argument. If that value is a promise object itself, then it's simply returned as is.
return Promise.resolve(item.value).then(
(v) => step("next", v),
(e) => step("throw", e)
);
} catch (e) {
reject(e);
return;
}
}
return step("next");
});
};
}
_asyncToGenerator(function* () { // <<< Now it's a generator function.
const foo = yield 3; // <<< Now it's yield, not await.
const bar = yield new Promise((resolve, reject) => resolve(7)); // <<< Each item is converted to a thenable object and recursively enclosed into chained then() calls.
const baz = bar * foo;
console.log(baz);
})();

Better understanding javascript's yield

I have the following code in my Koa app:
exports.home = function *(next){
yield save('bar')
}
var save = function(what){
var response = redis.save('foo', what)
return response
}
But I get the following error: TypeError: You may only yield a function, promise, generator, array, or object, but the following object was passed: "OK"
Now, "ok" is the response from the redis server, which makes sense. But I cannot fully grasp the concept of generators for this kinds of functions. Any help?
You don't yield save('bar') because SAVE is synchronous. (Are you sure you want to use save?)
Since it's synchronous, you should change this:
exports.home = function *(next){
yield save('bar')
}
to this:
exports.home = function *(next){
save('bar')
}
and it will block execution until it's finished.
Almost all other Redis methods are asynchronous, so you would need to yield them.
For example:
exports.home = function *(next){
var result = yield redis.set('foo', 'bar')
}
Yield is supposed to be used inside a generator function according to the documentation.
The purpose is to return the result of an iteration to be used in the next iteration.
Like in this example (taken from the documentation):
function* foo(){
var index = 0;
while (index <= 2) // when index reaches 3,
// yield's done will be true
// and its value will be undefined;
yield index++;
}
var iterator = foo();
console.log(iterator.next()); // { value:0, done:false }
console.log(iterator.next()); // { value:1, done:false }
console.log(iterator.next()); // { value:2, done:false }
console.log(iterator.next()); // { value:undefined, done:true }

Generator returns undefined for a value while waiting on promise to resolve

I have a function connectImpl referenced in multiple places. I am trying to invoke this promise and return its value out to the calling function synchronously through intermediating via a generator. If I call .next() on the generator it is returned in a pending state
{ value: { state: 'pending' }, done: false }
I would like to wait on the value of this generator until it is no longer pending. I have tried multiple versions of waitOn to accomplish this, but I cannot seem to make it work properly.
I am open to implementation suggestions. This is driving me a bit batty. Surprisingly the final created log of the promise fires later in the execution chain - after the generator is done. I am obviously missing something:
let models = null ;
let connectImpl = function() {
console.log('connectImpl')
let orm = setupImpl()
let config = getConfigImpl()
let qInitialize = q.nbind(orm.initialize, orm)
if(models) {
console.log('connectImpl:cached')
return q(models)
} else {
console.log('connectImpl:create')
return qInitialize(config).then(function(m){
console.log('connectImpl:created')
models = m
return models
})
}
}
let waitOn = function(generator){
console.log('waitOn')
let done = false ;
let generatorValue = null
while(!done){
var generatorResult = generator.next()
console.log(generatorResult)
done = generatorResult.done
generatorValue = generatorResult.value
}
return generatorValue
}
let domainImpl = function() {
console.log('domainImpl')
let getConnection = function *() {
console.log('domainImpl:getConnection')
yield connectImpl()
}
var generator = getConnection()
return waitOn(generator)
}
console.log('START')
console.log(domainImpl())
console.log('END')
I am able to invoke and get the
START
domainImpl
waitOn
domainImpl:getConnection
connectImpl
connectImpl:create
{ value: { state: 'pending' }, done: false }
{ value: undefined, done: true }
undefined
END
connectImpl:created
I am able to add execute the connectImpl promise to work with the middleware via this function - but I can't seem to adapt this to my above use case:
let domainMiddlewareImpl = function () {
return function *(next) {
let models = yield connectImpl()
this.request.models = models.collections;
this.request.connections = models.connections;
yield next
};
};
This looks fun. Let's see how we can yield promises. Our end goal is to write something like:
waitOn(function*(){
console.log("hello");
yield Q.delay(2000); // a placeholder, your calls in your example
console.log("World"); // this should run two seconds late.
});
Your issue here is that you're yielding them without waiting for them in advance. First of all, you can skip to the end for a 'ready' solution (don't!) and here is a fiddle of what we're making. Let's go through implementing waitOn with generators:
Let's start:
function waitOn(gen){
}
So, our function takes a generator, the first thing we'll have to do is invoke it since we need to execute the generator to get its results:
function waitOn(gen){
let sequence = gen(); // call the generator
}
Next, we'll want to wrap everything in a Promise since our waitOn will yield promises and return a promise for being done itself:
function waitOn(gen){
let sequence = gen(); // call the generator
return Promise.resolve(); // this is Q.resolve with Q
}
Now, what cases do we have:
The generator is done and returned a value - that is a return
The generator yielded a regular value and we do not have to wait for it
The generator yielded a promise and we have to wait for it. We also have to deal with exceptions (what if we yield a promise that rejects?)
So our basic structure is something like:
function waitOn(gen){
let sequence = gen(); // call the generator
return Promise.resolve().then(function cont(value){
let {value, done} = en.next(value); // get the next item
// depending on the case do what's appropriate
});
}
Note the destructuring assignment - I assume that's ok since your code has ES6 statements in it too. Note since this is the first call, value is undefined but generally we'll want to pass the value from our last call on. Now to handle the cases:
function waitOn(gen){
let sequence = gen(); // call the generator
return Promise.resolve().then(function cont(value){
let {done, value} = en.next(value); // get the next item
if(done) return value; // return case
if(!value || !value.then) return cont(value); // value case, recurse
return value.catch(e => gen.throw(e)).then(cont); // promise case
});
}
Note the .catch clause - we're throwing our code from the promise back to the generator for it to handle so we can try/catch the promises.
That's it! In 9 lines of JavaScript we've implemented generators for promises. Now to your code, you can yield any promise:
let conn = q.nBind(orm.initialize, orm);
waitOn(function*(){
console.log("Starting")
let handle = yield conn(config);
console.log("Handle created!", handle); // connected here
});
Happy coding and enjoy the power of coroutines. After we've had our fun - it's worth mentioning that Q already ships with Q.async and other newer promise libraries like Bluebird ship with their own (Bluebird has Promise.coroutine). If you're using a promise library - you can utilise those. This implementation works with native promises too.

Categories