Optional Map

OptionalMap.swift

//: Playground - noun: a place where people can play

import Cocoa

/* If you’re doing something like parsing JSON, and you have an array of
dictionaries, optionalMap is nice to only parse *valid* model objects out of
the dictionary. It allows you to be type-safe while still discarding junk
data. */
extension Array {
    func optionalMap<U>(transform: (T) -> U?) -> [U] {
        var result: [U] = []
        for item in self {
            if let transformedItem = transform(item) {
                result.append(transformedItem)
            }
        }
        return result
    }
}

struct foo: DebugPrintable {
    
    let name: String
    let id: UInt
    
    init?(dictionaryRepresentation: [String: AnyObject]) {
        if let name = dictionaryRepresentation["name"] as? String,
            id = dictionaryRepresentation["id"] as? UInt {
                self.name = name
                self.id = id
        }
        else {
            // There’s a bug in the Swift compiler that won’t let you return nil
            // without initializing all required properties.
            self.name = ""
            self.id = 0
            return nil
        }
    }
    
    var debugDescription: String {
        return "(Name: \(name), ID: \(id))"
    }
    
}

let parsedJSON = [["name": "firstFoo", "id": 0], ["name": "secondFoo", "id": 1],
    ["name": "invalidFoo"]]

let validFoos = parsedJSON.optionalMap { foo(dictionaryRepresentation: $0) }

println(validFoos.debugDescription)
// Prints: [(Name: firstFoo, ID: 0), (Name: secondFoo, ID: 1)]