sequence expressions allows you to do things in sequence.

A sequence expression is built up from multiple parts:

  • expressions or statements that return a Promise or a Result

  • catch branches

  • an optional finally branch

An example of handling loading something with a request:

sequence {
/* Started to load the users */
next { loading = true }
Make the request and wait for it to complete
and store in the "response" variable
response =
|> Http.send()
/* Parse the body of the response as JSON */
body =
|> Maybe.toResult("Json Parse Error")
Try to decode the list of users and convert the
result into a Promise so we can handle it here then
store it in the "users" variable
users =
decode response.body as Array(User)
/* If everything went well store the users */
next { users = users }
/* If the request fails handle it here */
} catch Http.Error => error {
next { error = "Something went wrong loading the request." }
/* If the decoding fails handle it here */
} catch Object.Error => error {
next { error = Object.Error.toString(error) }
/* Catch everything else */
} catch {
next { error = "Could not decode the response." }
After everything is handled or finished
it's not loading anymore
} finally {
next { loading = false }

Keep in mind that you need to handle all possible errors that can be returned from a statement, although the compiler has your back here and will show an error if you forget one.

The return value of a sequence expression is always a Promise(error, result) :

  • If the last statement is a Promise then it will be returned as is

  • If the last statement is not a Promise then it will return a promise which never fails Promise(Never, result) where the result is the value of the last statement

A few notable things to keep in mind:

  • All of the catches must match the return value of the last statement

  • Results and unwrapped into variables

  • Promises are await -ed and unwrapped into variables