lustre/cmd

Types

Cmd

A Cmd represents some side effect we want the Lustre runtime to perform. It is parameterised by our app’s action type because some effects need to get information back into your program.

pub opaque type Cmd(action)

Functions

batch

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pub fn batch(cmds: List(Cmd(a))) -> Cmd(a)

from

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pub fn from(cmd: fn(fn(a) -> Nil) -> Nil) -> Cmd(a)

Create a Cmd from some custom side effect. This is mostly useful for package authors, or for integrating other libraries into your Lustre app.

We pass in a function that recieves a dispatch callback that can be used to send messages to the Lustre runtime. We could, for example, create a tick command that uses the setTimeout JavaScript API to send a message to the runtime every second:

import lustre/cmd.{Cmd}

external fn set_interval(callback: fn() -> any, interval: Int) =
  "" "window.setInterval"

pub fn every_second(msg: msg) -> Cmd(msg) {
  use dispatch <- cmd.from

  set_interval(fn() { dispatch(msg) }, 1000)
}

map

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pub fn map(cmd: Cmd(a), f: fn(a) -> b) -> Cmd(b)

none

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pub fn none() -> Cmd(a)

Typically our app’s update function needs to return a tuple of #(model, Cmd(action)). When we don’t need to perform any side effects we can just return none()!

to_list

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pub fn to_list(cmd: Cmd(a)) -> List(fn(fn(a) -> Nil) -> Nil)