Getting Started: Input And Output

Input and Output

A standard API for input and output, known as "HSL IO", is included in the Hack Standard Library (HSL).

HSL IO should not yet be used in Facebook www; you want the Facebook-specific Filesystem class instead. Post in the usual groups if you can't find a suitable alternative for HSL IO.

HSL IO differs from most other language's standard IO libraries in two particularly significant ways:

  • provide as much safety as possible through the type system instead of runtime checking. For example, files opened read-only are a different type to those opened write-only; read-write files are a supertype of both read-only- and write-only- files.
  • designed primarily for asynchronous IO; blocking operations are not generally exposed.

Additional design goals include:

  • be internally consistent
  • reduce end-user boilerplate. For example, automatically retry calls that fail with EAGAIN
  • make the most obvious way to do things as safe as possible
  • /support/ all reasonable behavior even if unsafe
  • be convenient for common cases, but not at the cost of consistency or safety

For a more detailed overview, see the documentation for IO\Handle; basic operations include:

use namespace HH\Lib\{File, IO};

async function main_async(): Awaitable<void> {
  // STDIN for CLI, or HTTP POST data
  $_in = IO\request_input();
  // STDOUT for CLI, or HTTP response
  $out = IO\request_output();

  $message = "Hello, world\n";
  await $out->writeAllAsync($message);

  // copy to a temporary file, automatically closed at scope exit
  using ($tf = File\temporary_file()) {
    $f = $tf->getHandle();

    await $f->writeAllAsync($message);

    $content = await $f->readAllAsync();
    await $out->writeAllAsync($content);
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