Erica's code challenge

ShowPrices.swift

// For https://gist.github.com/erica/5477d5caccfbff04a802
// MARK: Generic collections

import Foundation
import Swift

infix operator ➜ { associativity left }

func ➜(source: [String], f: (String) -> ()) -> () {
    for element in source {
        f(element)
    }
}

func ➜<T, U>(source: [T], f: (T) -> U?) -> [U] {
    var results: [U] = []
    for element in source {
        if let result = f(element) {
            results.append(result)
        }
    }
    return results
}

func ➜<T, U>(source: [T], f: (T) -> (() -> U?)) -> [U] {
    return source ➜ { f($0) }
}

// MARK: JSONs

// This is incomplete! We just test the syntax.

extension NSURL {
    func fetchJSON() -> JSON? {
        return JSON()
    }
}

class JSON {
    
    func arrayValue(key:String) -> [JSON] {
        return []
    }
    
    func stringValue(key:String) -> String? {
        return ""
    }

    func intValue(key:String) -> Int? {
        return 0
    }
}

func usingFirst<T>(extractor: JSON -> (() -> T?)) -> (JSON -> T?) {
    return { json in 
        if let result = json.arrayValue("results").first {
            return extractor(result)()
        }
        return nil;
    }
}

// MARK: Specific models 
struct Price {
    let name: String
    let formattedPrice: String
}

struct Rating {
    let name: String
    let stars: Int?
}

private extension JSON {
    
    func extractPrice() -> Price? {
        if let name   = stringValue("trackName"), 
               price  = stringValue("formattedPrice") 
        {
            return Price(name: name, formattedPrice: price)
        }
        return nil;
    }
    
    func extractRating() -> Rating? {
        if let name = stringValue("trackName") {
            // Rating keys vary by content type
            return Rating(name: name, stars: intValue("averageUserRating") 
                                          ?? intValue("trackContentRating") )
        }
        return nil
    }
}

// MARK: Query URL
func FetchQueryURL(string: String) ->  NSURL? {
    return NSURL(string: string)
}

// MARK: Main functions
func ShowPrices(productURLStrings: [String]) {
    println("Today's prices")
    
    productURLStrings 
        ➜ FetchQueryURL  
        ➜ NSURL.fetchJSON      
        ➜ usingFirst(JSON.extractPrice) 
        ➜ { "\($0.name): \($0.formattedPrice)" }
        ➜ println
    
    println()
}

func CheckRatings(productURLStrings: [String]) {
    println("Checking ratings")
    
//    productURLStrings
//        ➜ FetchQueryURL 
//        ➜ NSURL.fetchJSON 
//        ➜ usingFirst(JSON.extractRating)    
//        ➜ { "\($0.name): \($0.starDescription)" } 
//        ➜ println
//    

    productURLStrings
        ➜ FetchQueryURL 
        ➜ NSURL.fetchJSON 
        ➜ usingFirst(JSON.extractRating)    

        // More complex blocks than { "\($0.name): \($0.starDescription)" } can be supplied.
        // Although for some reason Xcode is not able to infer this closure's type.
        ➜ { (rating: Rating) -> String? in 
            var desc = "ratings not available"
            map(rating.stars) { desc = "\($0) stars" }
            return rating.name + " " + desc
        }
        
        ➜ println
    
    println()
}