JSON Parsing in Swift

parsing.swift

// This code accompanies a blog post: http://chris.eidhof.nl/posts/json-parsing-in-swift.html
//
// As of Beta5, the >>= operator is already defined, so I changed it to >>>=    


import Foundation

let parsedJSON : [String:AnyObject] = [
    "stat": "ok",
    "blogs": [
        "blog": [
            [
                "id" : 73,
                "name" : "Bloxus test",
                "needspassword" : true,
                "url" : "http://remote.bloxus.com/"
            ],
            [
                "id" : 74,
                "name" : "Manila Test",
                "needspassword" : false,
                "url" : "http://flickrtest1.userland.com/"
            ]
        ]
    ]
]

struct Blog {
    let id: Int
    let name: String
    let needsPassword : Bool
    let url: NSURL
}

func parseBlog(blog: AnyObject) -> Blog? {
    let mkBlog = curry {id, name, needsPassword, url in Blog(id: id, name: name, needsPassword: needsPassword, url: url) }

    return asDict(blog) >>>= {
        mkBlog <*> int($0,"id")
            <*> string($0,"name")
            <*> bool($0,"needspassword")
            <*> (string($0, "url") >>>= toURL)
    }
}

func parseJSON() {
    let blogs = dictionary(parsedJSON, "blogs") >>>= {
        array($0, "blog") >>>= {
            join($0.map(parseBlog))
        }
    }
    println("posts: \(blogs)")
}

extension Blog : Printable {
    var description : String {
        return "Blog { id = \(id), name = \(name), needsPassword = \(needsPassword), url = \(url)"
    }
}

func toURL(urlString: String) -> NSURL {
    return NSURL(string: urlString)
}


func asDict(x: AnyObject) -> [String:AnyObject]? {
    return x as? [String:AnyObject]
}


func join<A>(elements: [A?]) -> [A]? {
    var result : [A] = []
    for element in elements {
        if let x = element {
            result += [x]
        } else {
            return nil
        }
    }
    return result
}



infix operator  <*> { associativity left precedence 150 }
func <*><A, B>(l: (A -> B)?, r: A?) -> B? {
    if let l1 = l {
        if let r1 = r {
            return l1(r1)
        }
    }
    return nil
}

func flatten<A>(x: A??) -> A? {
    if let y = x { return y }
    return nil
}

func array(input: [String:AnyObject], key: String) ->  [AnyObject]? {
    let maybeAny : AnyObject? = input[key]
    return maybeAny >>>= { $0 as? [AnyObject] }
}

func dictionary(input: [String:AnyObject], key: String) ->  [String:AnyObject]? {
    return input[key] >>>= { $0 as? [String:AnyObject] }
}

func string(input: [String:AnyObject], key: String) -> String? {
    return input[key] >>>= { $0 as? String }
}

func number(input: [NSObject:AnyObject], key: String) -> NSNumber? {
    return input[key] >>>= { $0 as? NSNumber }
}

func int(input: [NSObject:AnyObject], key: String) -> Int? {
    return number(input,key).map { $0.integerValue }
}

func bool(input: [NSObject:AnyObject], key: String) -> Bool? {
    return number(input,key).map { $0.boolValue }
}


func curry<A,B,R>(f: (A,B) -> R) -> A -> B -> R {
    return { a in { b in f(a,b) } }
}

func curry<A,B,C,R>(f: (A,B,C) -> R) -> A -> B -> C -> R {
    return { a in { b in {c in f(a,b,c) } } }
}

func curry<A,B,C,D,R>(f: (A,B,C,D) -> R) -> A -> B -> C -> D -> R {
    return { a in { b in { c in { d in f(a,b,c,d) } } } }
}

infix operator  >>>= {}

func >>>= <A,B> (optional : A?, f : A -> B?) -> B? {
    return flatten(optional.map(f))
}

// This code is provided under the MIT license:
// Copyright (c) 2014 Chris Eidhof
// 
// Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:
// 
// The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.
// 
// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.