Alternate if-let escape patterns (compare http://ericasadun.com/2015/02/16/swift-linearizing-code/)

gistfile1.swift

import Foundation

func BuildError(val: Int, msg: String) -> NSError { return NSError() }

func Error<T>(errorPointer: NSErrorPointer, val: Int, msg: String) -> T? {
    if errorPointer != nil {
        errorPointer.memory = BuildError(val, msg)
    }
    return nil
}

// This is the style I've been using. Note that I just pass errorPointer along to NSData()
// I don't create a new Error? variable
func StringFromURL(url : NSURL, error errorPointer : NSErrorPointer) -> NSString? {
    if let
        data = NSData(contentsOfURL:url, options: NSDataReadingOptions(rawValue:0), error: errorPointer) {
            return NSString(data: data, encoding: NSUTF8StringEncoding)
                ?? Error(errorPointer, 1, "Unable to build string from data")
    } else {
        return nil
    }
}

// I haven't really tried this style yet, but with Swift 1.2's multi-let,
// this is probably a nicer idea, and I'll probably try it in the future.
// See how ?? Error() basically converts a non-NSErrorPointer function into an NSErrorPointer function
func StringFromURL2(url : NSURL, error errorPointer : NSErrorPointer) -> NSString? {
    if let
        data = NSData(contentsOfURL:url, options: NSDataReadingOptions(rawValue:0), error: errorPointer),
        string = NSString(data: data, encoding: NSUTF8StringEncoding)
            ?? Error(errorPointer, 1, "Unable to build string from data") {
                return string
    }
    return nil
}


// This is a new style I've been exploring that exploits Swift 1.2's
// new feature that a let must be assigned to exactly once. This provides
// nice error checking against accidentally forgetting to set result, or
// setting it multiple times.
func StringFromURL3(url : NSURL, error errorPointer : NSErrorPointer) -> NSString? {
    let result: NSString?

    if let
        data = NSData(contentsOfURL:url, options: NSDataReadingOptions(rawValue:0), error: errorPointer) {
            result = NSString(data: data, encoding: NSUTF8StringEncoding)
                ?? Error(errorPointer, 1, "Unable to build string from data")
    } else {
        result = nil
    }

    return result
}