Promises+Filter.swift

import Promises

/// Check whether the values fulfill condition, and group the results.
/// - parameters:
///  - values: Values to check
///  - condition: Promise which states the condition
/// - returns: Promise which has grouped result of condition-passed values
public func filter<T>(values: [T], condition: ((T) -> Promise<Bool>)) -> Promise<[T]> {
    let promises = values.map { value in condition(value).validate { $0 }.then { _ in value } }
    return any(promises).then { $0.compactMap({ $0.value }) }
}