Note: Elm has managed effects, meaning that things like HTTP requests or writing to disk are all treated as data in Elm. When this data is given to the Elm runtime system, it can do some “query optimization” before actually performing the effect. Perhaps unexpectedly, this managed effects idea is the heart of why Elm is so nice for testing, reuse, reproducibility, etc.
Elm has two kinds of managed effects: commands and subscriptions.
When you need to subscribe to multiple things, you can create a batch of
subscriptions.
Note: Sub.none and Sub.batch [ Sub.none, Sub.none ] and
Sub.batch [] all do the same thing.
A subscription is a way of telling Elm, “Hey, let me know if anything interesting happens over there!” So if you want to listen for messages on a web socket, you would tell Elm to create a subscription. If you want to get clock ticks, you would tell Elm to subscribe to that. The cool thing here is that this means Elm manages all the details of subscriptions instead of you. So if a web socket goes down, you do not need to manually reconnect with an exponential backoff strategy, Elm does this all for you behind the scenes!
Every
Subspecifies (1) which effects you need access to and (2) the type of messages that will come back into your application.Note: Do not worry if this seems confusing at first! As with every Elm user ever, subscriptions will make more sense as you work through the Elm Architecture Tutorial and see how they fit into a real application!