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.
There are two kinds of managed effects you will use in your programs: commands and subscriptions.
A command is a way of telling Elm, “Hey, I want you to do this thing!” So if you want to send an HTTP request, you would need to command Elm to do it. Or if you wanted to ask for geolocation, you would need to command Elm to go get it.
Every
Cmdspecifies (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, commands will make more sense as you work through the Elm Architecture Tutorial and see how they fit into a real application!