├── 1Sets.playground
    ├── Contents.swift
    ├── contents.xcplayground
    └── playground.xcworkspace
    │   ├── contents.xcworkspacedata
    │   └── xcuserdata
    │       └── grahamlee.xcuserdatad
    │           └── UserInterfaceState.xcuserstate
├── 2Lists.playground
    ├── Contents.swift
    ├── contents.xcplayground
    └── playground.xcworkspace
    │   ├── contents.xcworkspacedata
    │   └── xcuserdata
    │       └── grahamlee.xcuserdatad
    │           └── UserInterfaceState.xcuserstate
├── 3DynamicDispatch.playground
    ├── Contents.swift
    ├── contents.xcplayground
    └── playground.xcworkspace
    │   ├── contents.xcworkspacedata
    │   └── xcuserdata
    │       └── grahamlee.xcuserdatad
    │           └── UserInterfaceState.xcuserstate
├── 4ArbitrarySelectors.playground
    ├── Contents.swift
    └── contents.xcplayground
└── README


/1Sets.playground/Contents.swift:
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 1 | import Cocoa
 2 | 
 3 | // let's define a set as a container in which an object is either present or absent.
 4 | // here's a definition of what a set of integers might look like:
 5 | typealias IntegerSet = (Int) -> Bool
 6 | typealias MySet<T> = (T) -> Bool
 7 | 
 8 | // so these two things are both examples, or instances, of the "Set of Integers" idea
 9 | let emptySet : MySet<Int> = {(_) in return false}
10 | let universe : MySet<Int> = {(_) in return true}
11 | 
12 | emptySet(12)
13 | universe(12)
14 | 
15 | // but they're pretty unexciting. it'd be nice to have some _configurable_ Sets, so we
16 | // can decide what is or is not in a Set. We'll create a function that returns sets, and
17 | // we can configure them by passing parameters to the functions.
18 | 
19 | func RangeSet<T: Comparable>(from lower: T, to upper: T) -> MySet<T>
20 | {
21 |     return { (x) in (x >= lower) && (x <= upper)}
22 | }
23 | 
24 | // notice that instances of RangeSet are implemented as _closures_ over the parameters in
25 | // the constructor.
26 | 
27 | let threeFourFive : IntegerSet = RangeSet(from: 3, to: 5)
28 | 
29 | threeFourFive(2)
30 | threeFourFive(3)
31 | 
32 | let oneAndTwo : IntegerSet = RangeSet(from: 1, to: 2)
33 | 
34 | // we can build set definitions out of other sets, too
35 | // notice here that the two sets escape the constructor, not the instance: we have
36 | // instance variable encapsulation
37 | func UnionSet<T>(of left: @escaping MySet<T>,
38 |                  and right: @escaping MySet<T>) -> MySet<T>
39 | {
40 |     return { (x) in (left(x) || right(x))}
41 | }
42 | 
43 | let oneToFive : IntegerSet = UnionSet(of: oneAndTwo, and: threeFourFive)
44 | 
45 | oneToFive(0)
46 | oneToFive(6)
47 | oneToFive(2)
48 | 
49 | let twoToFour : IntegerSet = RangeSet(from: 2, to: 4)
50 | 
51 | func IntersectionSet<T>(of left: @escaping MySet<T>, and right: @escaping MySet<T>) -> MySet<T>
52 | {
53 |     return { (x) in (left(x) && right(x)) }
54 | }
55 | 
56 | // because our sets are all the same type but respond in the same way, we can use
57 | // different "types" of sets anywhere a set is expected.
58 | // This is polymorphism.
59 | let intersectional = IntersectionSet(of: oneToFive, and: twoToFour)
60 | 
61 | intersectional(2)
62 | intersectional(1)
63 | 
64 | 


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/1Sets.playground/contents.xcplayground:
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1 | <?xml version="1.0" encoding="UTF-8" standalone="yes"?>
2 | <playground version='5.0' target-platform='macos'>
3 |     <timeline fileName='timeline.xctimeline'/>
4 | </playground>


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/1Sets.playground/playground.xcworkspace/contents.xcworkspacedata:
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1 | <?xml version="1.0" encoding="UTF-8"?>
2 | <Workspace
3 |    version = "1.0">
4 |    <FileRef
5 |       location = "self:">
6 |    </FileRef>
7 | </Workspace>
8 | 


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/1Sets.playground/playground.xcworkspace/xcuserdata/grahamlee.xcuserdatad/UserInterfaceState.xcuserstate:
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/2Lists.playground/Contents.swift:
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 1 | import Cocoa
 2 | 
 3 | // what we saw in episode 1 was that sets can be defined as a single operation: given an element of
 4 | // the set's type, is that element a member of the set? That let us create a polymorphic set type,
 5 | // as anything with the signature (Element)->Bool is a set.
 6 | 
 7 | // But what if we have more than one operation? Consider a list, which has a count of elements and
 8 | // a way to retrieve an element in a given position. Now the definition of a list instance is not a
 9 | // single function, it's _two_ functions: the count and the retrieval function. Let's package those
10 | // up into a structure: anything that is an instance of that structure is a list, as far as we're
11 | // concerned.
12 | 
13 | struct List<T> {
14 |     let count: () -> Int
15 |     let at: (Int) -> T?
16 | }
17 | 
18 | // the members of the structure are _selectors_: given a list, we can "select" its count method or
19 | // its at method for execution. Here's a constructor for a boring list
20 | 
21 | func EmptyList<T>() -> List<T>
22 | {
23 |     return List(count: {return 0}, at: {_ in return nil})
24 | }
25 | 
26 | let noIntegers: List<Int> = EmptyList()
27 | 
28 | noIntegers.count()
29 | noIntegers.at(0)
30 | 
31 | // as with sets, we can build lists that close over their constructor parameters as instance variables.
32 | // the easiest way to build a list of arbitrary length is to build one with two members.
33 | func LinkedList<T>(head: T, tail: List<T>) -> List<T>
34 | {
35 |     return List(count: { return 1 + tail.count() },
36 |                 at: {index in return (index == 0) ? head : tail.at(index - 1)})
37 | }
38 | 
39 | let oneTwoThree : List<Int> = LinkedList(head: 1, tail: LinkedList(head: 2, tail: LinkedList(head: 3, tail: EmptyList())))
40 | 
41 | oneTwoThree.count()
42 | oneTwoThree.at(0)
43 | oneTwoThree.at(1)
44 | 
45 | // this is all very well, but imagine that I want to add a new behaviour to my lists, for example the
46 | // ability to produce a description. I can add that method to the list description:
47 | 
48 | struct DescribableList<T> {
49 |     let count: () -> Int
50 |     let at: (Int) -> T?
51 |     let describe: () -> String
52 | }
53 | 
54 | // but do I really want to go back and change every List I've ever created to make it a describable list?
55 | // let's look at an alternative: delegation. We'll make a selector table in which the `count` and `at`
56 | // selectors are _optional_: a DescribableList object can supply those methods, or rely on some other
57 | // object supplying them, such as an existing list.
58 | 
59 | struct DescribableListSelectors<T> {
60 |     let count: (() -> Int)?
61 |     let at: ((Int) -> T?)?
62 |     let describe: () -> String
63 | }
64 | 
65 | // and a constructor that takes a list instance and adds a description, optionally also overriding
66 | // the `count` and `at` methods. We don't _need_ to override them, we can _inherit_ them from the list.
67 | 
68 | func ListSubtypeByAddingDescription<T>(prototype: List<T>,
69 |                                        overrides: DescribableListSelectors<T>) -> DescribableList<T>
70 | {
71 |     let countImplementation: () -> Int = overrides.count ?? prototype.count
72 |     let atImplementation: (Int) -> T? = overrides.at ?? prototype.at
73 |     
74 |     return DescribableList(count: countImplementation,
75 |                            at: atImplementation,
76 |                            describe: overrides.describe)
77 | }
78 | 
79 | // so to add a description method to a list, we use that list as the prototype
80 | func ListOfStrings(strings: List<String>) -> DescribableList<String>
81 | {
82 |     let describe: () -> String = {
83 |         var output = ""
84 |         for i in 0..<strings.count() {
85 |             output = output.appending(strings.at(i)!)
86 |             output = output.appending(" ")
87 |         }
88 |         return output.trimmingCharacters(in: CharacterSet.whitespaces)
89 |     }
90 |     return ListSubtypeByAddingDescription(prototype: strings,
91 |                                           overrides: DescribableListSelectors(count: nil,
92 |                                                                               at: nil,
93 |                                                                               describe: describe))
94 | }
95 | 
96 | let greeting = ListOfStrings(strings: LinkedList(head: "Hello,", tail: LinkedList(head: "World!", tail: EmptyList())))
97 | greeting.describe()
98 | 


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/2Lists.playground/contents.xcplayground:
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1 | <?xml version="1.0" encoding="UTF-8" standalone="yes"?>
2 | <playground version='5.0' target-platform='macos'>
3 |     <timeline fileName='timeline.xctimeline'/>
4 | </playground>


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/2Lists.playground/playground.xcworkspace/contents.xcworkspacedata:
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1 | <?xml version="1.0" encoding="UTF-8"?>
2 | <Workspace
3 |    version = "1.0">
4 |    <FileRef
5 |       location = "self:">
6 |    </FileRef>
7 | </Workspace>
8 | 


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/2Lists.playground/playground.xcworkspace/xcuserdata/grahamlee.xcuserdatad/UserInterfaceState.xcuserstate:
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https://raw.githubusercontent.com/iamleeg/OOPInFPInSwift/117a74affdb04c5e47e94340abfb0e304cad03cd/2Lists.playground/playground.xcworkspace/xcuserdata/grahamlee.xcuserdatad/UserInterfaceState.xcuserstate


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/3DynamicDispatch.playground/Contents.swift:
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  1 | import Cocoa
  2 | 
  3 | // the Sets in episode one of this series had a single method, the membership test, so were defined
  4 | // exactly as a function that implemented the membership test.
  5 | 
  6 | // in episode two, Lists had two methods: counting and retrieval, so we used structures to implement
  7 | // tables that contained these two methods. This turned out to be quite cumbersome when we wanted
  8 | // to extend lists, because we had to create a different table with the two+others methods we now needed
  9 | // and some machinery to map between them.
 10 | 
 11 | // the functional programming feature for mapping between two things is a "function", so let's stop using
 12 | // tables and start using functions. We'll take baby steps, starting by just changing the table into a
 13 | // function. The selector mapping for a List could look like this:
 14 | 
 15 | enum ListSelectors {
 16 |     case count
 17 |     case at
 18 | }
 19 | 
 20 | // now rather than get a table containing _all_ methods, we'll be able to call a function with a selector
 21 | // get a _specific_ method. Notice that `count` and `at` have different signatures, so the selector map
 22 | // actually needs to be a union type: we need to understand the method signature for a selector
 23 | 
 24 | enum MyList<T> {
 25 |     case count (() -> Int)
 26 |     case at ((Int) -> T?)
 27 | }
 28 | 
 29 | // now an object can be written as a message dispatch function: given a message containing a selector
 30 | // find the method that corresponds to the selector.
 31 | 
 32 | func EmptyList<T>(_cmd: ListSelectors) -> MyList<T>
 33 | {
 34 |     switch _cmd {
 35 |     case .count:
 36 |         return MyList<T>.count({ return 0 })
 37 |     case .at:
 38 |         return MyList<T>.at({ _ in return nil })
 39 |     }
 40 | }
 41 | 
 42 | // because Swift requires you to use you're type's good, you need to make sure the type of the thing
 43 | // you extract from this discriminated union matches the type you put in. This makes invoking the
 44 | // returned method quite cumbersome:
 45 | 
 46 | let emptyCountIMP : MyList<Int> = EmptyList(_cmd: .count)
 47 | switch emptyCountIMP {
 48 | case .count(let f):
 49 |     f()
 50 | default:
 51 |     assertionFailure("Method signature does not match expected type for selector")
 52 | }
 53 | 
 54 | // Doing that every time would be tedious, so rather than make clients do message lookups and invoke the
 55 | // messages themselves over and over, we'll give them a "message send" syntax which takes care of dispatching
 56 | // the selector, making sure the method signature matches and invoking the discovered method.
 57 | func countOfList<T>(list: ((ListSelectors)->MyList<T>)) -> Int
 58 | {
 59 |     switch list(.count) {
 60 |     case .count(let f):
 61 |         return f()
 62 |     default:
 63 |         assertionFailure("Method signature does not match expected type for selector")
 64 |         // unreached
 65 |         return 0
 66 |     }
 67 | }
 68 | 
 69 | func objectInListAtIndex<T>(list: ((ListSelectors)->MyList<T>), index: Int) -> T?
 70 | {
 71 |     switch list(.at) {
 72 |     case .at(let f):
 73 |         return f(index)
 74 |     default:
 75 |         assertionFailure("Method signature does not match expected type for selector")
 76 |         // unreached
 77 |         return nil
 78 |     }
 79 | }
 80 | 
 81 | // so, given a linked list built from this system
 82 | func MyLinkedList<T>(head: T, tail: @escaping ((ListSelectors)->MyList<T>)) -> ((ListSelectors) -> MyList<T>)
 83 | {
 84 |     return {(_cmd) in
 85 |         switch _cmd {
 86 |         case .count:
 87 |             return MyList<T>.count({
 88 |                 return 1 + countOfList(list: tail)
 89 |             })
 90 |         case .at:
 91 |             return MyList<T>.at({ (i) in
 92 |                 if i < 0 { return nil }
 93 |                 if i == 0 { return head }
 94 |                 return objectInListAtIndex(list: tail, index: i - 1)
 95 |             })
 96 |         }
 97 |     }
 98 | }
 99 | 
100 | let unitList = MyLinkedList(head: "Hello",
101 |                             tail: MyLinkedList(head: "World",
102 |                                                tail: EmptyList))
103 | 
104 | // we can easily use its methods via the type-safe dispatch functions
105 | countOfList(list: unitList)
106 | objectInListAtIndex(list: unitList, index: 1)
107 | 
108 | 


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/3DynamicDispatch.playground/contents.xcplayground:
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1 | <?xml version="1.0" encoding="UTF-8" standalone="yes"?>
2 | <playground version='5.0' target-platform='macos'>
3 |     <timeline fileName='timeline.xctimeline'/>
4 | </playground>


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/3DynamicDispatch.playground/playground.xcworkspace/contents.xcworkspacedata:
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1 | <?xml version="1.0" encoding="UTF-8"?>
2 | <Workspace
3 |    version = "1.0">
4 |    <FileRef
5 |       location = "self:">
6 |    </FileRef>
7 | </Workspace>
8 | 


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/3DynamicDispatch.playground/playground.xcworkspace/xcuserdata/grahamlee.xcuserdatad/UserInterfaceState.xcuserstate:
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/4ArbitrarySelectors.playground/Contents.swift:
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  1 | import Cocoa
  2 | 
  3 | // in episode 3, we replaced the selector tables in our list type with dispatch functions that took a
  4 | // selector as an argument and returned a method implementation. However, we did not address a big
  5 | // shortcoming that we found in episode 2: lists are defined as "something that has a `count` method and
  6 | // an `at` method", and anything else - including things that have a `count` method, an `at` method, and
  7 | // a `describe` method - is not a list.
  8 | 
  9 | // the dispatch tables from episode 2, and the dispatch functions from episode 3, are _total_ functions
 10 | // over a limited space of possible selectors. In this episode, we'll expand the space of selectors to
 11 | // be any string
 12 | 
 13 | typealias Selector = String
 14 | 
 15 | // and we'll make method dispatch a _partial_ function so that we don't have to supply all infinity
 16 | // methods. As with episode 3, there are multiple possible method signatures for the method at a given
 17 | // selector, so we'll build a discriminated union of possible implementation types.
 18 | 
 19 | enum IMP {
 20 |     case accessor(()->((Selector)->IMP)?)
 21 |     case asInteger(()->Int?)
 22 |     case methodMissing(()->((Selector)->IMP)?)
 23 |     case mutator((((Selector)->IMP))->Void)
 24 |     case description(()->String?)
 25 | }
 26 | 
 27 | // most of these cases exist so that we can move data between this system and the Swift type system,
 28 | // with one important exception.
 29 | 
 30 | // Notice the repeated signature `(Selector)->IMP`. That comes up a few times: accessors and mutators treat
 31 | // that type as their return value and argument respectively, and also it can be returned in the case of
 32 | // a missing method, to allow the Selector to be passed to a handler that can forward the message or
 33 | // otherwise deal with it.
 34 | 
 35 | // My assertion is that what we're going to build in this episode are functions that turn selectors into
 36 | // method implementations. They can work with references to other functions of that type. The grand claim
 37 | // that this whole series is predicated around is the following type alias:
 38 | 
 39 | typealias Object = (Selector) -> IMP
 40 | 
 41 | // in other words, an object is nothing other than a function that maps messages onto methods.
 42 | 
 43 | // here's the empty object constructor.
 44 | func DoesNothing() -> Object {
 45 |     var _self : Object! = nil
 46 |     func myself (selector: Selector)->IMP {
 47 |         return IMP.methodMissing({assertionFailure("method missing: \(selector)"); return nil;})
 48 |     }
 49 |     _self = myself
 50 |     return _self
 51 | }
 52 | 
 53 | let o : Object = DoesNothing()
 54 | 
 55 | // here's something a bit more interesting: an object that captures a Swift `Int` and makes it part of our
 56 | // object system. It provides a method for retrieving the `Int`.
 57 | 
 58 | func Integer(x: Int, proto: @escaping Object) -> Object {
 59 |     var _self : Object! = nil
 60 |     let _x = x
 61 |     func myself(selector:Selector) -> IMP {
 62 |         switch(selector) {
 63 |         case "intValue":
 64 |             return IMP.asInteger({ return _x })
 65 |         case "description":
 66 |             return IMP.description({ return "\(_x)" })
 67 |         default:
 68 |             return IMP.methodMissing({ return proto })
 69 |         }
 70 |     }
 71 |     _self = myself
 72 |     return _self
 73 | }
 74 | 
 75 | let theMeaning = Integer(x:42, proto:o)
 76 | 
 77 | // we'll come back to that `proto` parameter shortly. Meanwhile, remember that in the previous episode we
 78 | // had to unpack all those enumerated types and discriminated unions. We have to do that here, too. Let's
 79 | // build the "message lookup" operator, which returns the method implementation for a given selector.
 80 | // If the receiving object doesn't implement the selector, it should return a `methodMissing` implementation
 81 | // which can tell us another object to ask.
 82 | 
 83 | infix operator ..
 84 | 
 85 | func .. (receiver: Object?, _cmd:Selector) -> IMP? {
 86 |     if let this = receiver {
 87 |         let method = this(_cmd)
 88 |         switch(method) {
 89 |         case .methodMissing(let f):
 90 |             return f().._cmd
 91 |         default:
 92 |             return method
 93 |         }
 94 |     }
 95 |     else {
 96 |         return nil
 97 |     }
 98 | }
 99 | 
100 | // so now we can look up methods on objects. Like Objective-C, if the receiver is `nil`, then the result
101 | // is `nil` (unlike Objective-C, that `nil` is the empty value of the target type, not a magic way of
102 | // handling a 0)
103 | nil.."intValue"
104 | 
105 | // notice that the `Integer` we created up there has a `proto` instance variable, and any method it doesn't
106 | // understand gets passed to `proto`. That's because this object system is more like Self or JavaScript than
107 | // Smalltalk or Objective-C: it doesn't have classes, but you can implement shared behaviour by putting it
108 | // in a prototype object (that's prototype in the inheritance sense, not the cloning sense).
109 | // You can build classes out of prototypes, but I'll leave that for you.
110 | 
111 | // back to the machinery that removes the boilerplate of unboxing all those enums, it could look like this:
112 | func ℹ︎(receiver:Object?, _cmd:Selector)->Int? {
113 |     if let imp = receiver.._cmd {
114 |         switch(imp) {
115 |         case .asInteger(let f):
116 |             return f()
117 |         default:
118 |             return nil
119 |         }
120 |     } else {
121 |         return nil
122 |     }
123 | }
124 | 
125 | ℹ︎(receiver: theMeaning, _cmd: "intValue")
126 | 
127 | // notice that we're going to need a different function like ℹ︎ for every type in the `IMP` enum, which you
128 | // won't be used to from other OOP systems. In Smalltalk, Ruby, and similar, everything is an object
129 | // so the question of "what type is it" doesn't arise. In something like Objective-C, it _does_ arise,
130 | // and is handled internally (though imperfectly) by the runtime library. In Swift we have to play within
131 | // its type system so we have to explicitly cope with whatever types we've created.
132 | 
133 | func 🖨(receiver:Object?, _cmd:Selector)->String? {
134 |     if let imp = receiver.._cmd {
135 |         switch(imp) {
136 |         case .description(let f):
137 |             return f()
138 |         default:
139 |             return nil
140 |         }
141 |     } else {
142 |         return nil
143 |     }
144 | }
145 | 
146 | 🖨(receiver: theMeaning, _cmd:"description")
147 | 
148 | 


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/4ArbitrarySelectors.playground/contents.xcplayground:
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1 | <?xml version="1.0" encoding="UTF-8" standalone="yes"?>
2 | <playground version='5.0' target-platform='macos'>
3 |     <timeline fileName='timeline.xctimeline'/>
4 | </playground>


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/README:
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 1 | # OOPInFPInSwift
 2 | 
 3 | This is a collection of playgrounds in which object-oriented
 4 | programming is built out of functions in Swift. It supplants
 5 | [Objective-Swift](https://github.com/iamleeg/Objective-Swift), which
 6 | was written such a long time ago (two years) that everything is broken
 7 | and horrible. LOL computers.
 8 | 
 9 | These playgrounds were made in Xcode 9, so if WWDC 2018 has already
10 | happened you probably can't use these any more either.
11 | 
12 | ## Usage
13 | 
14 | Load the playgrounds in Xcode. They have descriptive comments.
15 | 
16 | ## Licence
17 | 
18 | Unless overridden by a licence grant in the source files, all of the
19 | sources in this repository are Copyright (C) 2017 Graham Lee and
20 | licensed to you under the terms of the [MIT
21 | License](https://opensource.org/licenses/MIT).
22 | 


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