Handle

:::info[Note] This is a point-in-time snapshot of the API documentation from January 2026. Going forward, we will not be maintaining a public copy of these references, and recommend users to refer to the built-in signature helpers available in the Hack LSP instead for complete and up-to-date information. :::

An interface for an IO stream

For example, an IO handle might be attached to a file, a network socket, or just an in-memory buffer.

HSL IO handles can be thought of as having a combination of behaviors - some of which are mutually exclusive - which are reflected in more-specific interfaces; for example:

• Closeable
• Seekable
• Readable
• Writable

These can be combined to arbitrary interfaces; for example, if you are writing a function that writes some data, you may want to take a IO\WriteHandle - or, if you read, write, and seek, IO\SeekableReadWriteHandle; only specify Closeable if your code requires that the close method is defined.

Some types of handle imply these behaviors; for example, all File\Handles are IO\SeekableHandles.

You probably want to start with one of:

• File\open_read_only(), File\open_write_only(), or File\open_read_write()
• IO\pipe()
• IO\request_input(), IO\request_output(), or IO\request_error(); these used for all kinds of requests, including both HTTP and CLI requests.
• IO\server_output(), IO\server_error()
• TCP\connect_async() or TCP\Server
• Unix\connect_async(), or Unix\Server

Interface Synopsis

namespace HH\Lib\IO;

interface Handle {...}