Import async function

[moics30]

I need to import an async function but have an error because when the function is evaluated return a promise.
Is there any way to do that?

Thanks for advance.

[harrysarson]

Would it be possible to share a small code snippet showing what you are trying to do?

[moics30]

Sure!

const math = require('mathjs');
math.import({
  getExchangeRate: function () {
    return new Promise(resolve => resolve(3.45)); // e.g. Connect to database, or any other async request.
  },
});

const expr = "a = b*getExchangeRate()"; // expression to evaluate

When eval "getExchangeRate", the error occur because the get value is [object Promise]

Thanks for advance.

[josdejong]

@moics30 non of the functions of math.js support promises, so if one of the values of multiply is a promise, the function has no clue what to do. You will have to first resolve the promise, store the result in a variable, and use that in your calculations (clearly separate sync and async).

[atulmy]

I'm trying to do something like this:

parser.set('select', async params => {
  const count = await Model.count()

  return count
}

However, when calling the parser const response = parser.evaluate('select()') throws following error:

TypeError: Value has unknown type. Value: [object Promise]

Can we create async functions? Any help on this would be highly appreciated.

CC: @josdejong

[harrysarson]

Async functions are not something that mathjs supports. Saying that your example should work - I made a quick example: https://repl.it/repls/GargantuanAcclaimedCoins

const mathjs = require('mathjs');

const parser = new mathjs.parser();

parser.set('select', async params => {
  await Promise.resolve();
  return 17;
})

parser.evaluate('select()')
  .then(console.log);

which does what you would expect it to do :)

[atulmy]

That was super quick @harrysarson, thanks!

I tried your example and it does work. However in case of parser.evaluate('select() + 10') it actually fails with the same issue. Any idea how can we achieve that?

Updated example: https://repl.it/repls/UnselfishBrownAlphatest

[josdejong]

The parser has no support for async responses. An async function should be invoke with async (or by resolving the returned promise), and this is not what the parser does.

I think you can simply do something like this:

const mathjs = require('mathjs')

async function run () {
  const parser = new mathjs.parser()

  const count = await Model.count() // first resolve (async)
  parser.set('select', count)       // then set the results (sync)

  const result = parser.evaluate('select()')
  console.log(result)
}

run()

So first you resolve your async value and after that put the results in the parser.

Be careful in your examples not to run any evaluation before your await Model.count() is resolved, until then it will of course be undefined.

[atulmy]

Okay, will use a work around. Thanks for the quick help and building this awesome library @josdejong!

[josdejong]

I do not see this as a workaround but as a proper, valid solution. The Parser simply is not async, and adding support for it would not add anything. I don't think it would make your code simpler or better. It may even make it more tricky and complicated when you mix sync and async on various levels that way: as you can see from my example you have to await for the count to resolve before you can evaluate. It would be very easy to make the mistake to call set without await and then call evaluate as Harry demonstrates with his example :D

[josdejong]

Sorry my previous comment sounds a bit offensive, that was not my intention. Anyway, I think there is no need at this point to add async support and it's easy to write your own wrapper if needed. Will close this question now, feel free to reopen if needed.

[atulmy]

I think async support would really help when we are working with cases where user creates own formula with different params to it and which needs to be resolved from database.

[josdejong]

Atul can you give a few examples of your idea of using async functions inside the parser? Some use-cases that would be very hard, verbose, cumbersome, or simply impossible to achieve without having async support in mathjs itself?