using_statement's Introduction

using_statement

Function call that acts like a using statement.

With Deno:

import { using } from "https://deno.land/x/using_statement/mod.ts";

Or with Node:

npm install --save using-statement

Example

Before:

const camera = new Camera();
try {
  outputPicture(camera.takePictureSync());
} finally {
  camera.dispose();
}

After:

import { using } from "https://deno.land/x/using_statement/mod.ts";

using(new Camera(), (camera) => {
  outputPicture(camera.takePictureSync());
});

Features

Supports synchronous, asynchronous, and generator functions.
Handles exceptions to ensure the resource is properly disposed.
Accepts objects with a dispose(), close(), or unsubscribe() method.
Allows asynchronously disposing when using a synchronous or asynchronous function.

Examples

Setup:

// Camera.ts
export class Camera {
  takePictureSync() {
    // ...etc...
    return pictureData;
  }

  async takePicture() {
    // ...etc...
    return pictureData;
  }

  dispose() {
    // clean up the resource this class is holding
  }
}

Synchronous example:

import { using } from "https://deno.land/x/using_statement/mod.ts";
import { Camera } from "./Camera.ts";

using(new Camera(), (camera) => {
  const picture = camera.takePictureSync();
  outputPicture(picture); // some function that outputs the picture
});

// camera is disposed here

Asynchronous example:

import { using } from "https://deno.land/x/using_statement/mod.ts";
import { Camera } from "./Camera.ts";

await using(new Camera(), async (camera) => {
  const picture = await camera.takePicture();
  outputPicture(picture);
});

// camera is disposed here

Generator function example:

import { using } from "https://deno.land/x/using_statement/mod.ts";
import { Camera } from "./Camera.ts";

const picturesIterator = using(new Camera(), function* (camera) {
  for (let i = 0; i < 10; i++) {
    yield camera.takePictureSync();
  }
});

// camera is not disposed yet...

for (const picture of picturesIterator) {
  outputPicture(picture);
}

// camera is now disposed

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using_statement's Issues

Dispose multiple resources in a single statement

using(new Camera(), new Camera(), (cameraA, cameraB) => {
    outputPicture(cameraA.takePictureSync());
    outputPicture(cameraB.takePictureSync());
});

Edit: Should be #1 (comment)

This is inspired by https://github.com/tc39/proposal-explicit-resource-management

// avoids leaking `a` or `b` to outer scope
// ensures `b` is disposed before `a` in case `b` depends on `a`
// ensures `a` is disposed even if disposing `b` throws
try (const a = ..., b = ...) { 
  ...
}