What is the "extraction" syntax in transformation functions called?

[andwrobs]

Hi all,

In the VoiceMemos example (and elsewhere), mapping/transformation functions such as fromChildAction use syntax like the following:

action: /Action.voiceMemo(id:action:)

• What is the Swift language feature that enables us to capture the id:action: using that smooshed sort of syntax?
• What is this syntax actually doing behind the scenes if it's sugar?

Cheers!

[tgrapperon]

Hey @andwrobs! There are various things at play here, but you should have a look at CasePaths for more informations about the inner bits.

A CasePath is something similar to a KeyPath that is able to extract/embed associated values for a given enum's case.

You can form a CasePath using explicit extract and embed implementations, but the library is also able to perform some reflection magic and to create a complete CasePath from a function (AssociatedValue) -> Enum.

Enum's cases with associated values are promoted to static functions of this exact shape when used without argument. For example:

enum E {
  case s(String)
}

E.s is inferred as a function (String) -> E, because E.s evaluated with ("Something") creates the E.s("something") enum's value (I don't know the corresponding Swift Evolution reference).

So what you see in your example is a custom prefix operator. Namely /. This operator expects a (UUID, VoiceMemo.Action) -> Action suffix function, and you pass Action.voiceMemo(id:action:).
It creates the CasePath corresponding to this enum's case. In other words:

[ / + (UUID, VoiceMemo.Action) -> Action ] ~> CasePath.init(embed: (UUID, VoiceMemo.Action) -> Action) ~> CasePath<Action, VoiceMemo.Action>

I don't know if this is clear! There is indeed a bunch of medium/high level features involved for this to work, but CasePath's source is relatively short and you can probably get a better idea by looking at it (you don't need to understand the reflection implementation BTW! (I don't!))

[andwrobs]

clear top to bottom, helps connect a lot of the dots, and gives me plenty to lookup to learn more 🙏

ty ty for explaining @tgrapperon !