ELM : Send command on init

29 Nov 2018

In the last post ELM : pick random item from list we discussed about finding a random number on a button click. But that not that intuitive. we should have a placeholder text until the user clicks on the button for the first time. To avoid this we need to send the FindRandom Cmd on the init itself.

This post will explain how we can achieve this.

Single Cmd

The init function returns a tuple of model and a Cmd. its signature will look like init : Flags -> ( Model, Cmd Msg ) and since we don’t have any Cmd to perform there we usually give Cmd.none.

init : Flags -> ( Model, Cmd Msg )
init flags =
   ( initialModel, Cmd.none)

Just replacing Cmd.none with FindRandom won’t work since FindRandom is a Msg and as per init type signature it needs Cmd Msg. Thats where the elm/Task comes in. As per the elm doc a Task can be described as

asynchronous operations that may fail, like HTTP requests or writing to a database.

In our case we can use this to convert the Msg into Cmd Msg using Task.succeed & Task.perform.

Task.succeed will convert the Msg into a Task which will succeed immediately and the run the task using Task.perform and convert into Cmd using an identity function.

An identity function is a function which takes one argument and return the same.

identity : a -> a
identity a =
   a

If you are thinking why identity function, it means that what ever the result of the task will be the result of the Cmd.

ie., Task.perform identity takes the task produced by Task.succeed FindRandom and convert into Cmd msg.

Thus our init function becomes

init : Flags -> ( Model, Cmd Msg )
init flags =
   ( initialModel, Task.succeed FindRandom |> Task.perform identity  )

The updated running sample is on ellie-app

Multiple Cmd

Last section we saw how to perform a Cmd msg on init, but in some cases we might need to perform more than one Cmd msg on init. For this we can use Cmd.batch to do this.

init : Flags -> ( Model, Cmd Msg )
init flags =
   ( initialModel, Cmd.batch
                [ Random.generate NewId (Random.int 1000 10000)
                , Task.succeed (AddItem "Setup & provision production server") |> Task.perform identity
                , Task.succeed (AddItem "Setup CI") |> Task.perform identity
                , Task.succeed (AddItem "Setup Project Skeleton") |> Task.perform identity
                  ])

The above example is taken from revathskumar/estimate-helper

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