A command is a way of telling an EffectManager, “Hey, I want you to do this thing!”
So if you want to send an HTTP request, you would need to command an EffectManager to do it.
Or if you wanted to ask for geolocation, you would need to command an EffectManager to go
get it.
Every Cmd specifies (1) which effects you need access to and (2) the type of
messages that will come back into your application.
When you need the runtime system to perform a couple commands, you can batch them together. Each is handed to the runtime at the same time, and since each can perform arbitrary operations in the world, there are no ordering guarantees about the results.
If you are using a fractal approach where a Cmd can come from a child's update function, you need to map the command in order for it to produce an action that can be routed back to the child.
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Note: TEA has managed effects, meaning that things like HTTP requests or writing to disk are all treated as data in TEA. When this data is given to the TEA runtime system, it can do some “query optimization” before actually performing the effect. Perhaps unexpectedly, this managed effects idea is the heart of why TEA is so nice for testing, reuse, reproducibility, etc.
TEA has two kinds of managed effects: commands and subscriptions.