ActionPropertySnippet.swift

let username = MutableProperty("")

let mutableView = username
	.validate { $0.characters.count >= 3 ? .success() : .failure(FormError("At least three characters.")) }
	.validate { !$0.characters.contains("@") ? .success() : .failure(FormError("Contains invalid characters.")) }
	.map(forward: { $0.isEmpty ? "" : "\($0)@reactiveswift" },
	     attemptBackward: { Result($0.stripSuffix("@reactiveswift"), failWith: FormError("Missing `@reactiveswift` suffix.")) })

mutableView.validations.observeValues { print("error: \($0.error?.reason ?? "NoError")") }

mutableView.value = "ya" // At least three characters.
print("value: \(username.value)") // ""

mutableView.value = "anders@reactiveswift" // NoError
print("value: \(username.value)") // "anders"

mutableView.value = "anders@@reactiveswift" // Contains invalid characters.
print("value: \(username.value)") // "anders"

struct FormError: Error {
	let reason: String

	init(_ reason: String) {
		self.reason = reason
	}
}

extension String {
	func stripSuffix(_ suffix: String) -> String? {
		if let range = range(of: suffix) {
			return substring(with: startIndex ..< range.lowerBound)
		}

		return nil
	}
}