Transforming ResultType<Value: CollectionType, Error> item by item

ResultExtensions.swift

import Result

public extension ResultType where Value: CollectionType {
    /// Returns a new Result by mapping each of `Success`es’ values using `transform`, or re-wrapping `Failure`s’ errors.
    public func mapByItem<U>(@noescape transform: Value.Generator.Element -> U) -> Result<[U], Error> {
        return flatMapByItem { .Success(transform($0)) }
    }
    
    /// Returns the result of applying `transform` to each of `Success`es’ values, or re-wrapping `Failure`’s errors.
    public func flatMapByItem<U>(@noescape transform: (Value.Generator.Element) -> Result<U, Error>) -> Result<[U], Error> {
        return analysis(
            ifSuccess: {
                $0.map(transform).reduce(.Success([])) { (acc, result) in
                    guard let successes = acc.value, success = result.value else {
                        return acc
                    }
                    return .Success(successes + [success])
                }
            },
            ifFailure: Result<[U], Error>.Failure)
    }
}