Swift sorting on arbitrary types - swift

Question

I have a Set of instances of type Thingie, and I want to provide arrays of Thingies sorted on any property of Thingie. Some of the properties are Int, for instance, while others are String, and there could be others. So I wanted to create a sort routine that accepts a string as the name of the property and compares the two properties of two thingies to determine the order.
It seemed like a job for generics, and I'm getting close, but there's a hole.
Here's where I'm at right now:
func compare<T:Comparable>(lft: T, _ rgt: T) -> Bool {
return lft < rgt
}
func orderBy(sortField: String) -> [Thingie] {
let allArray = (self.thingies as NSSet).allObjects as! [Thingie]
//typealias T = the type of allArray[0][sortField]
// or maybe create an alias that conforms to a protocol:
//typealias T:Comparable = ?
return allArray.sort({(a, b) -> Bool in
return self.compare(a[sortField] as! T, b[sortField] as! T)
})
}
I created a compare function using generics, and invoke it in my sort routine. The catch is that AnyObject! will not work for my generic, so I need to cast the values returned from a[sortField] and b[sortField] to be of the same type. It doesn't even really matter what type as long as the compiler is happy that both values are of the same type and that it implements the Comparable protocol.
I figured a typealias would do the trick, but maybe there's a better way?
Side question: surely there's a better way to create the initial, unsorted array from the set without resorting to NSSet. A little hint would be welcome. [Solved that bit! Thanks, Oliver Atkinson!]
Here's a big 'ol chunk of code you can paste into a playground. It has three attempts at the orderBy implementation, each with a problem.
//: Playground - noun: a place where people can play
import Foundation
class Thingie: Hashable {
var data: [String: AnyObject]
var hashValue: Int
init(data: [String: AnyObject]) {
self.data = data
self.hashValue = (data["id"])!.hashValue
}
subscript(propName: String) -> AnyObject! {
return self.data[propName]
}
}
func ==(lhs: Thingie, rhs: Thingie) -> Bool {
return lhs.hashValue == rhs.hashValue
}
var thingies: Set = Set<Thingie>()
thingies.insert(Thingie(data: ["id": 2, "description": "two"]));
thingies.insert(Thingie(data: ["id": 11, "description": "eleven"]));
// attempt 1
// won't compile because '<' won't work when type is ambiguous e.g., AnyObject
func orderByField1(sortField: String) -> [Thingie] {
return thingies.sort { $0[sortField] < $1[sortField] }
}
// compare function that promises the compiler that the operands for < will be of the same type:
func compare<T:Comparable>(lft: T, _ rgt: T) -> Bool {
return lft < rgt
}
// attempt 2
// This compiles but will bomb at runtime if Thingie[sortField] is not a string
func orderByField2(sortField: String) -> [Thingie] {
return thingies.sort { compare($0[sortField] as! String, $1[sortField] as! String) }
}
// attempt 3
// Something like this would be ideal, but protocol Comparable can't be used like this.
// I suspect the underlying reason that Comparable can't be used as a type is the same thing preventing me from making this work.
func orderByField3(sortField: String) -> [Thingie] {
return thingies.sort { compare($0[sortField] as! Comparable, $1[sortField] as! Comparable) }
}
// tests - can't run until a compiling candidate is written, of course
// should return array with thingie id=2 first:
var thingieList: Array = orderByField2("id");
print(thingieList[0]["id"])
// should return array with thingie id=11 first:
var thingieList2: Array = orderByField2("description");
print(thingieList2[0]["id"])

Answer 1

My previous answer, though it works, does not make the most of the Swift's excellent type checker. It also switches between the types that can be used in one centralised place which limits extensibility to the framework owner.
The following approach solves these issues. (Please forgive me for not having the heart to delete my previous answer; let us say that it's limitations are instructive...)
As before, we'll start with the target API:
struct Thing : ThingType {
let properties: [String:Sortable]
subscript(key: String) -> Sortable? {
return properties[key]
}
}
let data: [[String:Sortable]] = [
["id": 1, "description": "one"],
["id": 2, "description": "two"],
["id": 3, "description": "three"],
["id": 4, "description": "four"],
["id": 4, "description": "four"]
]
var things = data.map(Thing.init)
things.sortInPlaceBy("id")
things
.map{ $0["id"]! } // [1, 2, 3, 4]
things.sortInPlaceBy("description")
things
.map{ $0["description"]! } // ["four", "one", "three", "two"]
To make this possible we must have this ThingType protocol and an extension to mutable collections (which will work for sets as well as arrays):
protocol ThingType {
subscript(_: String) -> Sortable? { get }
}
extension MutableCollectionType
where Index : RandomAccessIndexType, Generator.Element : ThingType
{
mutating func sortInPlaceBy(key: String, ascending: Bool = true) {
sortInPlace {
guard let lhs = $0[key], let rhs = $1[key] else {
return false // TODO: nil handling
}
guard let b = (try? lhs.isOrderedBefore(rhs, ascending: ascending)) else {
return false // TODO: handle SortableError
}
return b
}
}
}
Evidently, the whole idea revolves around this Sortable protocol:
protocol Sortable {
func isOrderedBefore(_: Sortable, ascending: Bool) throws -> Bool
}
... which can be conformed to independently by any type we want to work with:
import Foundation
extension NSNumber : Sortable {
func isOrderedBefore(other: Sortable, ascending: Bool) throws -> Bool {
try throwIfTypeNotEqualTo(other)
let f: (Double, Double) -> Bool = ascending ? (<) : (>)
return f(doubleValue, (other as! NSNumber).doubleValue)
}
}
extension NSString : Sortable {
func isOrderedBefore(other: Sortable, ascending: Bool) throws -> Bool {
try throwIfTypeNotEqualTo(other)
let f: (String, String) -> Bool = ascending ? (<) : (>)
return f(self as String, other as! String)
}
}
// TODO: make more types Sortable (including those that do not conform to NSObject or even AnyObject)!
This throwIfTypeNotEqualTo method is just a convenience extension of Sortable:
enum SortableError : ErrorType {
case TypesNotEqual
}
extension Sortable {
func throwIfTypeNotEqualTo(other: Sortable) throws {
guard other.dynamicType == self.dynamicType else {
throw SortableError.TypesNotEqual
}
}
}
And that's it. Now we can conform new types to Sortable even outside of the framework and the type checker is validating our [[String:Sortable]] source data at compile time. Also, if Thing is extended to conform to Hashable then Set<Thing> will also be sortable by key...
Note that, although Sortable is itself unconstrained (which is awesome), source data and Thing's properties can be constrained to dictionaries with NSObject or AnyObject values if required by making use of a protocol like:
protocol SortableNSObjectType : Sortable, NSObjectProtocol { }
... or more directly by declaring data and Thing's properties as:
let _: [String : protocol<Sortable, NSObjectProtocol>]

Answer 2

I don't know the implementation of Thingie but maybe you could provide more context.
You could however go for something like this
func orderBy(sortField: String) -> [Thingie] {
return thingies.allObjects.map { $0 as! Thingie }.sort { $0[sortField] < $1[sortField] }
}
If you could provide a playground example so I can provide further help.
Also why did you use NSSet rather than a swift Set? would that give you what you want
let thingies: Set = Set<Thingie>()
func orderBy(sortField: String) -> [Thingie] {
return thingies.sort { $0[sortField] < $1[sortField] }
}
edit:
The trouble is with swift's type safety - it requires you to know what types you are dealing with so that it can compile correctly - if you specify the actual type when you want to order the field you can get it to work as expected.
func orderByField<T: Comparable>(sortField: String, type: T.Type) -> [Thingie] {
return thingies.sort { ($0[sortField] as? T) < ($1[sortField] as? T) }
}
var thingieList: Array = orderByField("id", type: Int.self);
print(thingieList[0]["id"])
var thingieList2: Array = orderByField("description", type: String.self);
print(thingieList2[0]["id"])
The above will print 2 then 11 - if you wanted to get around this you could store your objects in a different struct and then you can sort the array of 'Things' on the variable.
e.g.
struct Thing {
let id: Int
let description: String
}
var data: [Thing] = [
Thing(id: 2, description: "two"),
Thing(id: 11, description: "eleven")
]
let first = data.sort { $0.id < $1.id }.first?.id
let second = data.sort { $0.description < $1.description }.first?.id
print(first)
print(second)
Which would achieve the same thing - 2 and 11
I would advise against using AnyObject where possible as its trying to cheat the compiler into telling it you don't care for its help.
Its an interesting problem though and I hope this helps you towards your solution.

Answer 3

I will start with the target API (ignoring conformance to Hashable as its addition wont change anything in what follows). So, let's say we'd like to be able to write the following:
var thingies = [
["id": 1, "description": "one"],
["id": 2, "description": "two"],
["id": 3, "description": "three"],
["id": 4, "description": "four"]
].map(Thingie.init)
thingies.sortInPlace{ $0["id"] < $1["id"] }
... and even:
thingies.sortInPlaceBy("id")
thingies
.map{ $0["id"]!.value } // [1, 2, 3, 4]
thingies.sortInPlaceBy("description")
thingies
.map{ $0["description"]!.value } // ["four", "one", "three", "two"]
Obviously, we'd need an extension of MutableCollectionType protocol along the lines of:
protocol ThingieDatumSubscriptable {
subscript(_: String) -> ThingieDatum? { get }
}
extension Thingie : ThingieDatumSubscriptable {}
extension MutableCollectionType
where Index : RandomAccessIndexType, Generator.Element : ThingieDatumSubscriptable
{
mutating func sortInPlaceBy(datumName: String, ascending: Bool = true) {
let f: (ThingieDatum?, ThingieDatum?) -> Bool = ascending ? (<) : (>)
sortInPlace{ f($0[datumName], $1[datumName]) }
}
}
This ThingieDatum would then be something like:
import Foundation
struct ThingieDatum : Comparable {
let type: AnyObject.Type
let value: AnyObject
let name: String
init(keyValuePair: (String, AnyObject)) {
name = keyValuePair.0
value = keyValuePair.1
type = keyValuePair.1.dynamicType
}
}
... and its conformance to Comparable implemented in some sort of pedestrian way as follows (unless we introduce more protocols):
func == (lhs: ThingieDatum, rhs: ThingieDatum) -> Bool {
guard lhs.name == rhs.name && lhs.type == rhs.type else {
return false
}
switch lhs.type {
// TODO: implement for other types
case is NSNumber.Type: return lhs.value as! NSNumber == rhs.value as! NSNumber
case is NSString.Type: return (lhs.value as! String) == (rhs.value as! String)
default: break
}
return false
}
func < (lhs: ThingieDatum, rhs: ThingieDatum) -> Bool {
assert(lhs.name == rhs.name && lhs.type == rhs.type)
switch lhs.type {
// TODO: implement for other types
case is NSNumber.Type: return (lhs.value as! NSNumber).doubleValue < (rhs.value as! NSNumber).doubleValue
case is NSString.Type: return (lhs.value as! String) < (rhs.value as! String)
default: break
}
return false
}
Armed with such a ThingieDatum we can finally work out the Thingie itself:
struct Thingie {
var data: [ThingieDatum]
init(_ data: [String: AnyObject]) {
self.data = data.map(ThingieDatum.init)
}
subscript(datumName: String) -> ThingieDatum? {
for datum in data where datum.name == datumName {
return datum
}
return nil
}
}
And although this is, of course, all meant as a fun exercise, it does work (copy and paste into the playground if you can work our the correct order of snippets)... To take this idea further, however, we would probably want to constrain ThingiDatum initialiser to a custom protocol (rather than AnyObject), which would guarantee comparability. We would then conform to that protocol with each type we want to work with instead of switching through those types in one centralised place...