Algebraic Effects

Evolution Pitches
24

You could build an even closer analog in terms of async, since with*Continuation gives you a delimited continuation for the task (albeit a one-shot continuation, unlike the multi-shot continuations that "pure" algebraic effect systems provide in functional languages). You could for example store the effect handler in task-local state, and have the effect operations be functions that use withCheckedContinuation to access the current handler and resume the task with the result:

class State {
  var value: Int
}

@TaskLocal var stateHandler: State?

// Run the block with a state handler
func with(state: State, _ body: () async throws -> R) rethrows -> R {
  stateHandler = state
  defer { stateHandler = nil }
  return try await body()
}

// Access the current state
var stateValue: Int {
  get async {
    return withCheckedContinuation { cc in
      cc.resume(returning: stateHandler!.value)
    }
  }
}
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