effection

effection

Effortlessly composable structured concurrency primitive for JavaScript

See examples

To run an example with NAME:

$ node -r ./tests/setup examples/NAME

Structured Concurrency and Effects

Note: For an general introduction to the concept of structured concurrency, and why it is so important, see this excellent primer on the subject by Nathaniel Smith.

There's an entire hive of bugs that occur when asynchronous processes outlive their welcome. The concept of structured concurrency eliminates these altogether by providing the following guarantees:

1. A process is considered pending (unfinished) while it is either running or when it has any child process that is pending.
2. If an error occurs in a process that is not caught, then that error propagates to the parent process.
3. If a process finishes in error or by halting, then all of its child process are immediately halted.

For example, if you have the following processes:

+ - parent
  |
  + --- child A
  |
  + --- child B

We can make the following assertions based on these guarantees :

a. If the code associated with parent finishes running, but either A or B are pending, then parent is still considered pending.

b. If parent is halted, then both A and B are halted.

c. If an error is raised in parent, then both A and B are halted.

d. if an error is raised in A, then that error is also raised in parent, and child B is halted.

scenario d is of particular importance. It means that if a child throws an error and its parent doesn't catch it, then all of its siblings are immediately halted.

Execution

The process primitive is the Execution. To create (and start) an Execution, use the fork function and pass it a generator. This simplest example waits for 1 second, then prints out "hello world" to the console.

import { fork, timeout } from 'effection';

let process = fork(function*() {
  yield timeout(1000);
  return 'hello world';
});

process.isRunning //=> true
// 1000ms passes
// process.isRunning //=> false
// process.result //=> 'hello world'

Child processes can be composed freely. So instead of yielding for 1000 ms, we could instead, yield 10 times for 100ms.

fork(function*() {
  yield function*() {
    for (let i = 0; i < 10; i++) {
      yield timeout(100);
    }
  }
  return 'hello world';
})

And in fact, processes can be easily and arbitrarly deeply nested:

let process = fork(function*() {
  return yield function*() {
    return yield function*() {
      return yield function*() {
        return 'hello world';
      }
    };
  };
});

process.isCompleted //=> true
process.result //=> "hello world"

In order to abstract a process so that it can take arguments, you can use the call function:

import { fork, timeout, call } from 'effection';

function* waitForSeconds(durationSeconds) {
  yield timeout(durationSeconds * 1000);
}

fork(function*() {
  yield call(waitforseconds, 10);
});

More likely though, you would want to define a higher-order function that took your argument and returned a generator:

function waitForSeconds(durationSeconds) {
  return function*() {
    yield timeout(durationSeconds * 1000);
  }
}

Asynchronous Execution

Sometimes you want to fork some processes in parallel and not necessarily block further execution on them. You still want the guarantees associated with structured concurrency however. For example, you might want to create a couple of different servers as part of your main process. To do this, you would use the fork method on the execution:

import { fork, fork } from 'effection';

fork(function*() {
  fork(createFileServer);
  fork(createHttpServer);
});

Even though it exits almost immediately, the main process is not considered completed until both servers shutdown. More importantly though, if we shutdown the main process, then both servers will be halted.

Development

yarn install

$ yarn

run tests:

$ yarn test