Promise integration

[kennyjwilli]

I am interested in integrating the Promise POC into my application using mathjs. Specifically, I would like to write an expression that concatenates a string together with a value returned from a promise-returning function. e.g., I would like to do something like this.

concat('prefix--', generate_id())
  prefix--1

I have taken the POC code and written an example JSFiddle here. The short is, it doesn't seem to exactly work, and I'd love some help understand why, and if it is possible to fix. A basic example, similar to the POC example does work.

concat('hello', getDelayedResult(' world')) 
     hello world

If I make the example slightly more complex by, say, having getDelayedResult return a number that I cast to a string, the promise gets embedded in the string.

concat('hello', format(getDelayedResult(1))) 
     hello[object Promise]

The example code wraps the format function in the same promise resolving code as concat, but it does not seem to work for format for some reason. I thought it could be due to the nesting (e.g., concat -> format -> getDelayedResult), but removing the outer concat shows that it is unique to format.

format(getDelayedResult(1)) 
     [object Promise]

And the same occurs with string.

string(getDelayedResult(1)) 
     [object Promise]

And just for completeness, I also tried with format with an object param. No dice.

print('pre-$id', {id: getDelayedResult(1)}) 
     pre-[object Promise]

Any thoughts on what's going on here and how I may be able to fix/workaround this?

---

Update: I found an interesting workaround -- JSFiddle. Essentially, if you update the type signature for the promise factory to ...Promise, it will work for some of the examples.

"concat('hello', format(getDelayedResult(1))) 
     hello[object Promise]"
"print('pre-$id', {id: getDelayedResult(1)}) 
     pre-[object Promise]"
"format(getDelayedResult(1)) 
     [object Promise]"
"concat('hello', getDelayedResult(' world')) 
     hello world"
"concat('hello', string(getDelayedResult(1))) 
     hello1"
"string(getDelayedResult(1)) 
     1"

[josdejong]

I remember there where some tricky edge cases with this async POC but I don't remember the details, see #2665 (comment)

It would be interesting to pick up async support again BTW :)

[kennyjwilli]

Fwiw, my use case is similar to others that have brought up async -- I'd like to expose functionality from database interactions, which are exclusively async.