SE-0216: User-defined dynamically callable types

[dan-zheng]

@Patrick_Smith

For JavaScript specifically, there’s a difference between calling a method and ‘detaching’ a method into a function and calling it. It loses the reference to the receiver ( this in JavaScript, self in Swift).

Thanks for bringing this up! Some people discussed JavaScript interopability (including the binding problem) in the pitch threads for @dynamicCallable (here and here).

A useful starting point for JavaScript interop is working through how to add callable behavior for a specific implementation (e.g. extending JSValue from JavaScriptCore with callable behavior).

JSValue in particular defines the following method for performing JavaScript method calls:

extension JSValue {
  func invokeMethod(_ method: String!, withArguments arguments: [Any]!) -> JSValue!
}

invokeMethod(_:withArguments:) | Apple Developer Documentation

At a glance, this seems exactly like something @dynamicMemberCallable can sugar.

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I wonder if there are other implementations of JavaScript interop besides JSValue; thinking about how to extend those would be useful too.