package eio

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Defines the base resource type.

Resources are typically operating-system provided resources such as open files and network sockets. However, they can also be pure OCaml resources (such as mocks) or wrappers (such as an encrypted flow that wraps an unencrypted OS flow).

A resource's type shows which interfaces it supports. For example, a [source | sink] t is a resource that can be used as a source or a sink.

If you are familiar with object types, this is roughly equivalent to the type <source; sink>. We avoid using object types here as some OCaml programmers find them confusing.


type ('t, -'tags) handler

A ('t, 'tags) handler can be used to look up the implementation for a type 't.

'tags is a phantom type to record which interfaces are supported.

Internally, a handler is a set of bindings.

type -'tags t =
  1. | T : ('t * ('t, 'tags) handler) -> 'tags t

A resource is a pair of a value and a handler for it.

Normally there will be convenience functions provided for using resources and you will not need to match on T yourself except when defining a new interface.

Defining new interfaces

These types and functions can be used to define new interfaces that others can implement.

When defining a new interface, you will typically provide:

  • The tags that indicate that the interface is supported (e.g. Flow.source_ty).
  • A convenience type to match all sub-types easily (e.g. Flow.source).
  • Functions allowing users to call the interface (e.g. Flow.single_read).
  • A module to let providers implement the interface (e.g. Flow.Pi).
type ('t, 'iface, 'tag) pi = ..

A provider interface describes an interface that a resource can implement.

  • 't is the type of the resource itself.
  • 'iface is the API that can be requested.
  • 'tag is the tag (or tags) indicating that the interface is supported.

For example, the value Close (of type (fd, fd -> unit, [> `Close]) pi) can be used with a resource backed by an fd, and which offers at least the `Close tag, to request its close function. Often, the API requested will be a module type, but it can be a single function as in this example.

type _ binding =
  1. | H : ('t, 'impl, 'tags) pi * 'impl -> 't binding

A binding H (pi, impl) says to use impl to implement pi.

For example: H (Close, M.close).

val handler : 't binding list -> ('t, _) handler

handler ops is a handler that looks up interfaces using the assoc list ops.

For example shutdown (module Foo) is a handler that handles the Close and Shutdown interfaces for resources of type Foo.t by using the Foo module:

let shutdown (type t) (module X : SHUTDOWN with type t = t) : (t, shutdown_ty) handler =
  handler [
    H (Close, X.close);
    H (Shutdown, (module X));

Be sure to give the return type explicitly, as this cannot be inferred.

val bindings : ('t, _) handler -> 't binding list

bindings (handler ops) = ops.

This is useful if you want to extend an interface and you already have a handler for that interface.

val get : ('t, 'tags) handler -> ('t, 'impl, 'tags) pi -> 'impl

get handler iface uses handler to get the implementation of iface.

For example:

let write (Resource.T (t, ops)) bufs =
  let module X = (val (Resource.get ops Sink)) in
  X.write t bufs
val get_opt : ('t, _) handler -> ('t, 'impl, _) pi -> 'impl option

get_opt is like get, but the handler need not have a compatible type. Instead, this performs a check at runtime and returns None if the interface is not supported.


Resources are usually attached to switches and closed automatically when the switch finishes. However, it can be useful to close them sooner in some cases.

type close_ty = [
  1. | `Close
type pi +=
  1. | Close : ('t, 't -> unit, [> close_ty ]) pi
val close : [> close_ty ] t -> unit

close t marks the resource as closed. It can no longer be used after this.

If t is already closed then this does nothing (it does not raise an exception).

Note: if an operation is currently in progress when this is called then it is not necessarily cancelled, and any underlying OS resource (such as a file descriptor) might not be closed immediately if other operations are using it. Closing a resource only prevents new operations from starting.