process

Types

CallError

An error returned when making a call to a process.

pub type CallError(msg) {
  CalleeDown(reason: Dynamic)
  CallTimeout
}

Constructors

```
CalleeDown(reason: Dynamic)
```
The process being called exited before it sent a response.
```
CallTimeout
```
The process being called did not response within the permitted amount of time.

Cancelled

</>

Values returned when a timer is cancelled.

pub type Cancelled {
  TimerNotFound
  Cancelled(time_remaining: Int)
}

Constructors

```
TimerNotFound
```
The timer could not be found. It likely has already triggered.
```
Cancelled(time_remaining: Int)
```
The timer was found and cancelled before it triggered.

The amount of remaining time before the timer was due to be triggered is returned in milliseconds.

ExitMessage

</>

pub type ExitMessage {
  ExitMessage(pid: Pid, reason: ExitReason)
}

Constructors

ExitMessage(pid: Pid, reason: ExitReason)

ExitReason

</>

pub type ExitReason {
  Normal
  Killed
  Abnormal(reason: String)
}

Constructors

```
Normal
```
```
Killed
```
```
Abnormal(reason: String)
```

A Pid (or Process identifier) is a reference to an Erlang process. Each process has a Pid and it is one of the lowest level building blocks of inter-process communication in the Erlang and Gleam OTP frameworks.

pub type Pid

ProcessDown

</>

A message received when a monitored process exits.

pub type ProcessDown {
  ProcessDown(pid: Pid, reason: Dynamic)
}

Constructors

```
ProcessDown(pid: Pid, reason: Dynamic)
```

ProcessMonitor

opaque </>

pub opaque type ProcessMonitor

Selector

</>

A type that enables a process to wait for messages from multiple Subjects at the same time, returning whichever message arrives first.

Used with the new_selector, selecting, and select functions.

Examples

> let int_subject = new_subject()
> let float_subject = new_subject()
> send(int_subject, 1)
>
> let selector =
>   new_selector()
>   |> selecting(int_subject, int.to_string)
>   |> selecting(float_subject, float.to_string)
>
> select(selector, 10)
Ok("1")

pub type Selector(payload)

Subject

opaque </>

A Subject is a value that processes can use to send and receive messages to and from each other in a well typed way.

Each subject is “owned” by the process that created it. Any process can use the send function to sent a message of the correct type to the process that owns the subject, and the owner can use the receive function or the Selector type to receive these messages.

The Subject type is similar to the “channel” types found in other languages and the “topic” concept found in some pub-sub systems.

Examples

let subject = new_subject()

// Send a message with the subject
send(subject, "Hello, Joe!")

// Receive the message
receive(subject, within: 10)

pub opaque type Subject(message)

Timer

</>

pub type Timer

Functions

call

</>

pub fn call(
  subject: Subject(a),
  make_request: fn(Subject(b)) -> a,
  within timeout: Int,
) -> b

Send a message to a process and wait for a reply.

If the receiving process exits or does not reply within the allowed amount of time the calling process crashes. If you wish an error to be returned instead see the try_call function.

cancel_timer

</>

pub fn cancel_timer(timer: Timer) -> Cancelled

Cancel a given timer, causing it not to trigger if it has not done already.

demonitor_process

</>

pub fn demonitor_process(monitor monitor: ProcessMonitor) -> Nil

Remove the monitor for a process so that when the monitor process exits a ProcessDown message is not sent to the monitoring process.

If the message has already been sent it is removed from the monitoring process’ mailbox.

flush_messages

</>

pub fn flush_messages() -> Nil

Discard all messages in the current process’ mailbox.

Warning: This function may cause other processes to crash if they sent a message to the current process and are waiting for a response, so use with caution.

is_alive

</>

pub fn is_alive(a: Pid) -> Bool

Check to see whether the process for a given Pid is alive.

See the Erlang documentation for more information.

kill

</>

pub fn kill(pid: Pid) -> Nil

Send an untrappable kill exit signal to the target process.

See the documentation for the Erlang erlang:exit function for more information.

link

</>

pub fn link(pid pid: Pid) -> Bool

Creates a link between the calling process and another process.

When a process crashes any linked processes will also crash. This is useful to ensure that groups of processes that depend on each other all either succeed or fail together.

Returns True if the link was created successfully, returns False if the process was not alive and as such could not be linked.

map_selector

</>

pub fn map_selector(a: Selector(a), b: fn(a) -> b) -> Selector(b)

Add a transformation function to a selector. When a message is received using this selector the transformation function is applied to the message.

This function can be used to change the type of messages received and may be useful when combined with the merge_selector function.

merge_selector

</>

pub fn merge_selector(
  a: Selector(a),
  b: Selector(a),
) -> Selector(a)

Merge one selector into another, producing a selector that contains the message handlers of both.

If a subject is handled by both selectors the handler function of the second selector is used.

monitor_process

</>

pub fn monitor_process(pid: Pid) -> ProcessMonitor

Start monitoring a process so that when the monitored process exits a message is sent to the monitoring process.

The message is only sent once, when the target process exits. If the process was not alive when this function is called the message will never be received.

The down message can be received with a Selector and the selecting_process_down function.

The process can be demonitored with the demonitor_process function.

named

</>

pub fn named(name: Atom) -> Result(Pid, Nil)

Look up a process by name, returning the pid if it exists.

new_selector

</>

pub fn new_selector() -> Selector(a)

Create a new Selector which can be used to receive messages on multiple Subjects at once.

new_subject

</>

pub fn new_subject() -> Subject(a)

Create a new Subject owned by the current process.

pid_from_dynamic

</>

pub fn pid_from_dynamic(
  from from: Dynamic,
) -> Result(Pid, List(DecodeError))

Checks to see whether a Dynamic value is a pid, and return the pid if it is.

Examples

import gleam/dynamic from_dynamic(dynamic.from(process.self())) Ok(process.self())

from_dynamic(dynamic.from(123)) Error([DecodeError(expected: “Pid”, found: “Int”, path: [])])

receive

</>

pub fn receive(
  from subject: Subject(a),
  within milliseconds: Int,
) -> Result(a, Nil)

Receive a message that has been sent to current process using the Subject.

If there is not an existing message for the Subject in the process’ mailbox or one does not arrive within the permitted timeout then the Error(Nil) is returned.

Only the process that is owner of the Subject can receive a message using it. If a process that does not own the Subject attempts to receive with it then it will not receive a message.

To wait for messages from multiple Subjects at the same time see the Selector type.

register

</>

pub fn register(pid: Pid, name: Atom) -> Result(Nil, Nil)

This function will return an error under the following conditions:

The process for the pid no longer exists.
The name has already been registered.
The process already has a name.
The name is the atom undefined, which is reserved by Erlang.

select

</>

pub fn select(
  from from: Selector(a),
  within within: Int,
) -> Result(a, Nil)

Receive a message that has been sent to current process using any of the Subjects that have been added to the Selector with the selecting function.

If there is not an existing message for the Selector in the process’ mailbox or one does not arrive within the permitted timeout then the Error(Nil) is returned.

Only the process that is owner of the Subjects can receive a message using them. If a process that does not own the a Subject attempts to receive with it then it will not receive a message.

To wait forever for the next message rather than for a limited amount of time see the select_forever function.

select_forever

</>

pub fn select_forever(from from: Selector(a)) -> a

Similar to the select function but will wait forever for a message to arrive rather than timing out after a specified amount of time.

selecting

</>

pub fn selecting(
  selector: Selector(a),
  for subject: Subject(b),
  mapping transform: fn(b) -> a,
) -> Selector(a)

Add a new Subject to the Selector to that it’s messages can be received.

The mapping function provided with the Subject can be used to convert the type of messages received using this Subject. This is useful for when you wish to add multiple Subjects to a Seletor when they have differing message types. If you do not wish to transform the incoming messages in any way then the identity function can be given.

selecting_anything

</>

pub fn selecting_anything(
  selector: Selector(a),
  mapping handler: fn(Dynamic) -> a,
) -> Selector(a)

Add a catch-all handler to a selector that will be used when no other handler in a selector is suitable for a given message.

This may be useful for when you want to ensure that any message in the inbox is handled, or when you need to handle messages from other BEAM languages which do not use subjects or record format messages.

selecting_process_down

</>

pub fn selecting_process_down(
  selector: Selector(a),
  monitor: ProcessMonitor,
  mapping: fn(ProcessDown) -> a,
) -> Selector(a)

Add a ProcessMonitor to a Selector so that the ProcessDown message can be received using the Selector and the select function.

selecting_record2

</>

pub fn selecting_record2(
  selector: Selector(a),
  tag: b,
  mapping transform: fn(Dynamic) -> a,
) -> Selector(a)

Add a handler to a selector for 2 element tuple messages with a given tag element in the first position.

Typically you want to use the selecting function with a Subject instead, but this function may be useful if you need to receive messages sent from other BEAM languages that do not use the Subject type.

selecting_record3

</>

pub fn selecting_record3(
  selector: Selector(a),
  tag: b,
  mapping transform: fn(Dynamic, Dynamic) -> a,
) -> Selector(a)

Add a handler to a selector for 3 element tuple messages with a given tag element in the first position.

selecting_record4

</>

pub fn selecting_record4(
  selector: Selector(a),
  tag: b,
  mapping transform: fn(Dynamic, Dynamic, Dynamic) -> a,
) -> Selector(a)

Add a handler to a selector for 4 element tuple messages with a given tag element in the first position.

selecting_record5

</>

pub fn selecting_record5(
  selector: Selector(a),
  tag: b,
  mapping transform: fn(Dynamic, Dynamic, Dynamic, Dynamic) -> a,
) -> Selector(a)

Add a handler to a selector for 5 element tuple messages with a given tag element in the first position.

selecting_record6

</>

pub fn selecting_record6(
  selector: Selector(a),
  tag: b,
  mapping transform: fn(
    Dynamic,
    Dynamic,
    Dynamic,
    Dynamic,
    Dynamic,
  ) -> a,
) -> Selector(a)

Add a handler to a selector for 6 element tuple messages with a given tag element in the first position.

selecting_record7

</>

pub fn selecting_record7(
  selector: Selector(a),
  tag: b,
  mapping transform: fn(
    Dynamic,
    Dynamic,
    Dynamic,
    Dynamic,
    Dynamic,
    Dynamic,
  ) -> a,
) -> Selector(a)

Add a handler to a selector for 7 element tuple messages with a given tag element in the first position.

selecting_record8

</>

pub fn selecting_record8(
  selector: Selector(a),
  tag: b,
  mapping transform: fn(
    Dynamic,
    Dynamic,
    Dynamic,
    Dynamic,
    Dynamic,
    Dynamic,
    Dynamic,
  ) -> a,
) -> Selector(a)

Add a handler to a selector for 8 element tuple messages with a given tag element in the first position.

selecting_trapped_exits

</>

pub fn selecting_trapped_exits(
  selector: Selector(a),
  handler: fn(ExitMessage) -> a,
) -> Selector(a)

Add a handler for trapped exit messages. In order for these messages to be sent to the process when a linked process exits the process must call the trap_exit beforehand.

self

</>

pub fn self() -> Pid

Get the Pid for the current process.

send

</>

pub fn send(subject: Subject(a), message: a) -> Nil

Send a message to a process using a Subject. The message must be of the type that the Subject accepts.

This function does not wait for the Subject owner process to call the receive function, instead it returns once the message has been placed in the process’ mailbox.

Ordering

If process P1 sends two messages to process P2 it is guaranteed that process P1 will receive the messages in the order they were sent.

If you wish to receive the messages in a different order you can send them on two different subjects and the receiver function can call the receive function for each subject in the desired order, or you can write some Erlang code to perform a selective receive.

Examples

let subject = new_subject()
send(subject, "Hello, Joe!")

send_abnormal_exit

</>

pub fn send_abnormal_exit(pid: Pid, reason: String) -> Nil

Sends an exit signal to a process, indicating that the process is to shut down due to an abnormal reason such as a failure.

See the Erlang documentation for more information.

send_after

</>

pub fn send_after(
  subject: Subject(a),
  delay: Int,
  message: a,
) -> Timer

Send a message over a channel after a specified number of milliseconds.

send_exit

</>

pub fn send_exit(to pid: Pid) -> Nil

Sends an exit signal to a process, indicating that the process is to shut down.

See the Erlang documentation for more information.

sleep

</>

pub fn sleep(a: Int) -> Nil

Suspends the process calling this function for the specified number of milliseconds.

sleep_forever

</>

pub fn sleep_forever() -> Nil

Suspends the process forever! This may be useful for suspending the main process in a Gleam program when it has no more work to do but we want other processes to continue to work.

start

</>

pub fn start(
  running implementation: fn() -> a,
  linked link: Bool,
) -> Pid

Create a new Erlang process that runs concurrently to the creator. In other languages this might be called a fibre, a green thread, or a coroutine.

If linked is True then the created process is linked to the creator process. When a process terminates an exit signal is sent to all other processes that are linked to it, causing the process to either terminate or have to handle the signal.

More can be read about processes and links in the Erlang documentation.

subject_owner

</>

pub fn subject_owner(subject: Subject(a)) -> Pid

Get the owner process for a Subject. This is the process that created the Subject and will receive messages sent with it.

trap_exits

</>

pub fn trap_exits(a: Bool) -> Nil

Set whether the current process is to trap exit signals or not.

When not trapping exits if a linked process crashes the exit signal propagates to the process which will also crash. This is the normal behaviour before this function is called.

When trapping exits (after this function is called) if a linked process crashes an exit message is sent to the process instead. These messages can be handled with the selecting_trapped_exits function.

try_call

</>

pub fn try_call(
  subject: Subject(a),
  make_request: fn(Subject(b)) -> a,
  within timeout: Int,
) -> Result(b, CallError(b))

Send a message to a process and wait for a reply.

If the receiving process exits or does not reply within the allowed amount of time then an error is returned.

unlink

</>

pub fn unlink(pid: Pid) -> Nil

Removes any existing link between the caller process and the target process.

unregister

</>

pub fn unregister(name: Atom) -> Result(Nil, Nil)

Un-register a process name, after which the process can no longer be looked up by that name, and both the name and the process can be re-used in other registrations.

It is possible to un-register process that are not from your application, including those from Erlang/OTP itself. This is not recommended and will likely result in undesirable behaviour and crashes.