EagerEffect.swift

public final class EagerEffect<A> {

    private var callbacks: [(A) -> Void] = []
    private var value: A? {
        didSet { value.map(notify) }
    }

    public init(run: @escaping (@escaping (A) -> Void) -> Void) {
        run { self.value = $0 }
    }

    public func run(_ observer: @escaping (A) -> Void) {
        value.map(observer)
        callbacks.append(observer)
    }

    private func notify(_ a: A) {
        callbacks.forEach { callback in
            callback(a)
        }
    }
}