More exploration of guard/try and crazy operator idea

gistfile1.swift

// This is pretty clean, but I often dislike chains of temporary variables
// They tend to lead to little bugs when you use the wrong one. Swift's warnings
// and 'let' reduce problems, though.
func pagesFromOpenSearchData(data: NSData) throws -> [Page] {
    let json = try NSJSONSerialization.JSONObjectWithData(data, options: NSJSONReadingOptions())

    guard      let array  = json as? [JSON]                 else { throw JSONError.BadArray(json) }
    guard case let value  = array[1] where array.count >= 2 else { throw JSONError.OutOfRange }
    guard      let titles = value as? [String]              else { throw JSONError.BadStringList(json) }

    return titles.map { Page(title: $0) }
}


// Crazy, but maybe brilliant, idea by @radexp
// https://twitter.com/radexp/status/608754347061706752
infix operator ?! {}
func ?! <T>(a: T?, e: ErrorType) throws -> T {
    if let a = a {
        return a
    } else {
        throw e
    }
}

// This abandons the step-by-step error checking, which is much of the point of the exercise,
// but is closer to how most people error check. Doesn't need any temporary vriables
func pagesFromOpenSearchData2(data: NSData) throws -> [Page] {
    let json = try NSJSONSerialization.JSONObjectWithData(data, options: NSJSONReadingOptions())

    return try
        (json as? [JSON])
            .flatMap { $0.count >= 1 ? $0[1] : nil }
            .flatMap { $0 as? [String] }
            .map { $0.map { Page(title: $0)}}
        ?! JSONError.BadJSON(json)
}