OK. How do you express those effects as outputs in the interface?
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My point is exactly that _you don’t need to_. Why is it important? The effects are an implementation detail.
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Trying to figure out how this is different from any object which performs effects when it e.g. receives a message.
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Is the significant point that all endpoints are expressed via signal APIs rather than via methods?
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Yes. Everything can be treated as a functional series of inputs to outputs. Embedded effects are encapsulated away.
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Here’s one of my favorite examples: github.com/ReactiveCocoa/
I just embedded an animation effect, but it’s the same values.
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Man, the embedding of effects makes me super duper uncomfortable. How do you test this stuff? Mocks?
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Generally, we don’t test any of the effects themselves—they’re usually untestable things. The signals are still testable.
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My concern is that there's tons of testable, pure behavior in github.com/ReactiveCocoa/ alongside untestable effects.
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If you're actually practicing functional core, imperative shell, then it's easy to be sure you've tested all pure behavior.
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And impure behavior is obviously separated in a way which provides pressure for you to minimize it.
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e.g. looking at github.com/garybernhardt/, it's clear where "the danger zone"; any logic in the shell is a clear red flag