Cannot compile async code with generators in Typescript

Question

Context

In my journey into Typescript, I was advised that blocking calls should never be done inside async code.
I'm also using generators, since they make directory traversal easy and avoid running out of stack space.

However, when I combine async code (in this case: readdir) with generators, the compiler complains that yield can only be used inside generators, leading me to think that the compiler is not able to combine closures, generators and async code, all in one go.

function *yyyymmGenerator(dir: string, props: Props) {
    const fs    = require("fs");
    const yyyy  = props.range.getUTCFullYear().toString();
    const mm    = props.range.getUTCMonth().toString();
    const start = `${yyyy}-${mm}`;

    const files = fs.readdir(dir, function(err, files) {
        for (let i = 0; i < files.length; i++) {
            const file: string = files[i];
            if (file.localeCompare(start) >= 0) {
                const d = `${dir}/${file}`;
                yield file;
            }
        }
    });
}

error TS1163: A 'yield' expression is only allowed in a generator body.

Questions

What would be the recommended best practice in a situation like this?

Would it be OK if I simply consider everything sync, blocking code but "wrap" the call inside a Promise?

See stackoverflow.com/questions/41326217/…

Ryan Cavanaugh
– Ryan Cavanaugh

2017-01-24 18:12:04 +00:00
Commented Jan 24, 2017 at 18:12 — Ryan Cavanaugh
– Ryan Cavanaugh, Commented Jan 24, 2017 at 18:12

Olivier Boissé · Accepted Answer · 2017-01-23 16:43:32Z

3

You could do things like that (call next function inside the callback) :

const gen = yyyymmGenerator(args);

function callback(err, files) {
   if(err) return gen.throw(err);
   gen.next(files);
   // Print all files returned by the generator
   for(file of gen) {
     console.log(file);
   }
}

function *yyyymmGenerator(dir: string, props: Props) {
   const fs    = require("fs");
   const yyyy  = props.range.getUTCFullYear().toString();
   const mm    = props.range.getUTCMonth().toString();
   const start = `${yyyy}-${mm}`;

   const files = yield fs.readdir(dir, callback);

   for (let i = 0; i < files.length; i++) {
      const file: string = files[i];
      if (file.localeCompare(start) >= 0) {
         const d = `${dir}/${file}`;
         yield file;
      }
   }
}

gen.next(); // Start generator

edited Jan 23, 2017 at 16:43

answered Jan 23, 2017 at 16:31

Olivier Boissé

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2 Comments

Richard Gomes Over a year ago

I understand the solution you proposed and I think it would work, in principle. However, I would like to understand if I should (or could?) consider the error message an evidence that the compiler is limited in regards to the scopes involved. Or even if it could be considered a bug in the compiler. I would like to file an issue but first I would like to understand if the compiler complains by fault or by design. Any ideas?

Olivier Boissé Over a year ago

The compiler complains because you used yield outside a function* (in a callback), and this is logical because when your async function is called, the generator's execution doesn't stop, in your example the generator would probably be terminated before the execution of the readdir's callback

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