When Iterate is not clear enough ....

[Maatary]

Generates a potentially infinite stream of promises by repeatedly calling f until predicate becomes true.

Where:

f - function that, given a seed, returns the next value or a promise for it. predicate - function that receives the current iteration value, and should return truthy when the unfold should stop ***handler - function that receives each value as it is produced by f. It may return a promise to delay the next iteration. seed - initial value provided to the handler, and first f invocation. May be a promise.

The question is what is handler for exactly ?

In my case:

1 - i have a table of values

2 - for each value i need to check if a value X, is not related to the value of the tables.

2.2 - In that respect obtaining the related element of a value requires making an AJAX call.

2.3 - Then upon getting the result of related value, i check if the value X is not in that result.

2.4 - If it is i'm done. If not i repeat the task with the next value in the table.

I have the feeling that iterate is suited for that.

1 - However i don't understand how the handler can help me.

2 - I feel one could give as a seed, an object {table: tab, iter: 0}, then the next function would simply change the iter value and return the object.

3 - The issue is with predicate and handler.

3.1 - I need to make an ajax call, get my result and compare. Can the result of predicate be a promise ?

[Maatary]

Just for the record, what is "require" for ?

[Maatary]

It is not clear to me what is the result of iterate, in the sense that when iterate stops, what argument is passed to done(). What it deferred resolve to ?

[briancavalier]

Hey @Maatary. I'll try to answer some of your questions.

In general, when.iterate is for iteratively computing intermediate steps along the way to a final result. Each step may have some side effect, which is performed by the handler.

Does that help?

Just for the record, what is "require" for ?

Are you asking about the require() function, for example here?

what argument is passed to done()

The promise returned by when.iterate represents the value computed by the final iteration, that is, the final call to f. If you observe that promise by calling then or done on it, it will provide the final value to the callback you supply. For example:

when.iterate(...).then(function(x) {
    // x is the value from the final iteration of when.iterate
});

If you use done with no arguments, then you will not be able to observe the final result. That might be fine for your use case if you just care about the side effects that handler performs during the iterations. In that case, adding done() with no arguments at the end will simply cause a loud failure plus a stack trace if the iteration fails, which can be helpful for debugging.

If you haven't already read then vs. done, or the debugging promises section, you may find them to be helpful.

Hope that helps!

[Maatary]

Could you let me know how with iterate I could solve my use case. The handler to not return any value but perform a side effect. How can I know that I need to stop iterating based on what the handler compute.

On each element of the table I want to know if A is a narrower concept or a broader concept. So for each element I need to make an Ajax call get the narrower or broader and compare. If I find A in the list of broader or narrower I stop and return the element itself (or true) else I return a failure (or false).

So the result of is_broader or is_narrower are promises. That either true or false / element or failure.

Depending on their result I want to continue up the chain or not. And get either true/the element or false/failure.

So long as handler is for side effect I don't see how I can impact the iteration. On Sat, May 16, 2015 at 4:09 PM Brian Cavalier notifications@github.com wrote:

Hey @Maatary https://github.com/Maatary. I'll try to answer some of your questions.

In general, when.iterate is for iteratively computing intermediate steps along the way to a final result. Each step may have some side effect, which is performed by the handler.

Does that help?

Just for the record, what is "require" for ?

Are you asking about the require() function, for example here https://github.com/cujojs/when/blob/master/when.js#L12?

what argument is passed to done()

The promise returned by when.iterate represents the value computed by the final iteration, that is, the final call to f. If you observe that promise by calling then or done on it, it will provide the final value to the callback you supply. For example:

when.iterate(...).then(function(x) { // x is the value from the final iteration of when.iterate });

If you use done with no arguments, then you will not be able to observe the final result. That might be fine for your use case if you just care about the side effects that handler performs during the iterations. In that case, adding done() with no arguments at the end will simply cause a loud failure plus a stack trace if the iteration fails, which can be helpful for debugging.

If you haven't already read then vs. done https://github.com/cujojs/when/blob/master/docs/api.md#promisethen-vs-promisedone, or the debugging promises section https://github.com/cujojs/when/blob/master/docs/api.md#debugging-promises, you may find them to be helpful.

Hope that helps!

— Reply to this email directly or view it on GitHub https://github.com/cujojs/when/issues/454#issuecomment-102675472.