Brandon Kase: Grokking Lazy Sequences and Collections

1. GROKKING LAZY SEQUENCES AND COLLECTIONS By / Brandon Kase @bkase_
2. PHOTOS http://friendlycomputersspokane.com/wp-content/uploads/2014/06/photo-stack.jpg
3. PHOTOS
4. PROBLEMS There are a lot of photos Loading a photo's data is expensive
5. PROBLEMS There are a lot of things Loading a thing is expensive
6. HOLDING THINGS http://odditymall.com/includes/content/wall-mounted-hands-for-holding-stuff-0.jpg
7. HOLDING THINGS Arrays?
8. HOLDING THINGS let fetchresult = PHAsset.fetchAllAssets/*...*/()
9. HOLDING THINGS let fetchresult = PHAsset.fetchAllAssets/*...*/() var arr = [PHAsset]()
10. HOLDING THINGS let fetchresult = PHAsset.fetchAllAssets/*...*/() var arr = [PHAsset]() fetchresult.enumerateAssetsUsingBlock { obj, /*...*/ arr.append(obj as! PHAsset) }
11. HOLDING THINGS let fetchresult = PHAsset.fetchAllAssets/*...*/() var arr = [PHAsset]() fetchresult.enumerateAssetsUsingBlock { obj, /*...*/ arr.append(obj as! PHAsset) } // slowwwwww
12. HOLDING THINGS We need a Collection
13. GROUPS OF THINGS A Collection is an Indexable Sequence A Sequence is an Iterator maker An Iterator can produce one or more Elements
14. ITERATORPROTOCOL protocol IteratorProtocol { associatedtype Element mutating func next() -> Element? }
15. ITERATORPROTOCOL Let's say we want to iterate over the even numbers https://i.ytimg.com/vi/A9NfHIIwNN4/maxresdefault.jpg
16. ITERATORPROTOCOL struct Evens: IteratorProtocol { var state: Int = 0 mutating func next() -> Int? { let curr = state state += 2 return .some(curr) } }
17. ITERATORPROTOCOL let s = Evens() for _ in 1...5 { print(s.next()!) // 0, 2, 4, 6, 8 }
18. SEQUENCE Recall: A Sequence is an Iterator maker http://ichef.bbci.co.uk/news/660/media/images/80295000/jpg/_80295405_thinking522738989.jpg
19. ITERATORPROTOCOL struct Evens: IteratorProtocol, Sequence
20. SEQUENCE for x in Evens().prefix(5) { print(x) // 0, 2, 4, 6, 8 }
21. SEQUENCE http://customer-blog-images.s3.amazonaws.com/questions.jpg
22. SEQUENCE extension Sequence where Iterator == Self, Self: IteratorProtocol { func makeIterator() -> Iterator { return self } }
23. SEQUENCE http://www.thedirtondusters.com/wp-content/uploads/2012/08/WHAT-ELSE-NO-TEXT.jpg
24. SEQUENCE for x in someSequence { print(x) }
25. SEQUENCE map, filter, reduce prefix, dropFirst much, much more
26. SEQUENCE must be able to produce an iterator could be one-pass or multipass could be finite or infinite
27. COLLECTION http://ichef.bbci.co.uk/news/660/media/images/80295000/jpg/_80295405_thinking522738989.jpg
28. COLLECTION protocol Collection: Sequence, Indexable
29. COLLECTION http://friendlycomputersspokane.com/wp-content/uploads/2014/06/photo-stack.jpg
30. COLLECTION struct PhotosMetadata: Collection { /*...*/ var startIndex: Int { /*...*/ } var endIndex: Int { /*...*/ } func index(after i: Int) -> Int { /*...*/ } subscript(i: Int) -> PHAsset { /*...*/ } }
31. COLLECTION Why do we need to deal with the index crap? struct PhotosMetadata: Collection { /*...*/ var startIndex: Int { /*...*/ } var endIndex: Int { /*...*/ } func index(after i: Int) -> Int { /*...*/ } subscript(i: Int) -> PHAsset { /*...*/ } }
32. COLLECTION RULES has addressable positions (index) must support multipass iteration exists a one-to-one mapping of indices to elements No infinite structures! expect O(1) subscript access
33. COLLECTION Non-integer indices are useful: https://upload.wikimedia.org/wikipedia/commons/thumb/d/d4/CPT-LinkedLists-deletingnode.svg/380px-CPT- LinkedLists-deletingnode.svg.png
34. COLLECTION struct PhotosMetadata: Collection {
35. COLLECTION private let base: PHFetchResult init(_ base: PHFetchResult) { self.base = base }
36. COLLECTION var startIndex: Int { return 0 }
37. COLLECTION var endIndex: Int { return base.count }
38. COLLECTION func index(after i: Int) -> Int { return i + 1 }
39. COLLECTION subscript(i: Int) -> PHAsset { return base[i] as! PHAsset }
40. PHOTOS let a = PhotosMetadata(PHAsset.fetchAllAssets/*...*/()) // not slow!
41. PHOTOS Now we want to turn our PHAssets into Photos
42. PHOTOS struct Photo { let url: NSURL /*...*/ }
43. PHOTOS let metadata: PhotosMetadata let photos = metadata.map{ Photo(url: $0.url) } // slowwww
44. PHOTOS Delay metadata load
45. PHOTOS Delay operations on collections or sequences
46. LAZY GROUPS OF THINGS Transformations computed when the information is forced out https://media.giphy.com/media/AyXMnDH4nA7jW/giphy.gif
47. LAZYSEQUENCE protocol LazySequenceProtocol: Sequence
48. LAZYCOLLECTION protocol LazyCollectionProtocol: LazySequenceProtocol, Collection
49. NORMAL MAP // eager sequence/collection [1,2,3].map{ $0 + 1 } // [2, 3, 4]
50. LAZYSEQUENCE MAP Evens().lazy.map{ $0 + 1 } // LazyMapSequence<Int, Int> // nothing is computed yet!
51. LAZYCOLLECTION MAP [1,2,3].lazy.map{ $0 + 1 } // LazyMapRandomAccessCollection<Int, Int> // nothing is computed yet!
52. WAIT A SECOND protocol LazyCollectionProtocol: LazySequenceProtocol, Collection
53. H O W C A N A N O V E R R I D E N M E T H O D R E T U R N D I F F E R E N T T Y P E S ?
54. PROTOCOL DEFAULT IMPLEMENTATIONS https://pptcrafter.files.wordpress.com/2013/01/gears-0.png
55. DEFAULT IMPLEMENTATIONS protocol A { } extension A { func woo() -> Self { return self } func hi() -> String { return "A" } }
56. DEFAULT IMPLEMENTATIONS struct ConcA: A {} print(ConcA().woo().hi()) // "A"
57. DEFAULT IMPLEMENTATIONS protocol B { } extension B { func hi() -> Int { return 0 } }
58. DEFAULT IMPLEMENTATIONS struct Mystery: A, B { } print(Mystery().woo().hi()) // compile error!
59. DEFAULT IMPLEMENTATIONS // now B subsumes A protocol B: A { } extension B { func hi() -> Int { return 0 } }
60. DEFAULT IMPLEMENTATIONS struct Mystery: B { } print(Mystery().woo().hi()) // 0 // NOT a compile error!
61. MAPS Evens().lazy.map{ $0 + 1 } // LazyMapSequence<Int, Int> // nothing is computed yet!
62. LAZYMAPSEQUENCE? struct LazyMapSequence<S: Sequence>: LazySequenceProtocol
63. LAZYMAPSEQUENCE struct LazyMapSequence<S: Sequence>: LazySequenceProtocol { func makeIterator() -> LazyMapIterator</*...*/> { /*...*/ } }
64. LAZYMAPITERATOR struct LazyMapIterator<Base: Iterator, Element>: IteratorProtocol, Sequence { }
65. LAZYMAPITERATOR struct LazyMapIterator<Base: Iterator, Element>: IteratorProtocol, Sequence { var _base: Base let _transform: (Base.Element) -> Element }
66. LAZYMAPITERATOR struct LazyMapIterator<Base: Iterator, Element>: IteratorProtocol, Sequence { var _base: Base let _transform: (Base.Element) -> Element mutating func next() -> Element? { return _base.next().map(_transform) } }
67. LAZYMAPSEQUENCE struct LazyMapSequence<S: Sequence>: LazySequenceProtocol http://www.tacobueno.com/media/1381/beefbob.png?quality=65
68. BURRITOS let a = [1,2,3].lazy.map{ $0 + 1 }.filter{ $0 != 3 } // a: LazyFilterBidirectionalCollection< // LazyMapRandomAccessCollection< // Array<Int>, Int // > // >
69. BURRITOS http://images.wisegeek.com/flour-tortilla.jpg
70. PHOTOS let metadata: PhotosMetadata let photos = metadata.lazy.map{ Photo(url: $0.url) } // not slow!
71. PHOTOS func prepareData(d: PhotosMetadata) -> ? { return d.lazy .filter{ $0.isPhoto() } // skip videos .map{ Photo(url: $0.url) } // make photos .map{ PhotoView(photo: $0) } // make view }
72. PHOTOS LazyFilterBidirectionalCollection< LazyMapRandomAccessCollection< LazyMapRandomAccessCollection< PHAsset, Photo >, PhotoView > >
73. TORTILLA INFERENCE http://m.jumpstart.com/JumpstartNew/uploadedFiles/sne/small-screenshots/the-magic-tortilla.jpg
74. TYPE INFERENCE let m = d.lazy.map/*...*/ return m.first
75. TORTILLA ERASURE https://2e0a24317f4a9294563f-26c3b154822345d9dde0204930c49e9c.ssl.cf1.rackcdn.com/9218426_the-tortilla- pencil-case-deliciously-wraps_11d463da_m.jpg?bg=DFC1B7
76. TYPE ERASURE let m = AnyCollection( d.lazy.filter{}.map{}/*...*/ ) // m: AnyCollection<PhotoView>
77. TYPE ERASURE AnySequence AnyCollection ... see appendix for more
78. PHOTOS func prepareData(d: PhotosMetadata) -> AnyCollection<PhotoView>
79. PHOTOS Product wants everyOther photo http://www.giftofcuriosity.com/wp-content/uploads/2016/04/Letter-hopscotch-4.jpg
80. NEW TORTILLAS let a = [1,2,3,4] for x in a.lazy.everyOther() { print(x) // 2,4 }
81. NEW TORTILLAS // the tortilla sequence struct LazyEveryOtherSequence <S: Sequence>: LazySequenceProtocol { var base: S func makeIterator() -> LazyEveryOtherIterator<S.Iterator> { return LazyEveryOtherIterator(base: base.makeIterator()) } }
82. NEW TORTILLAS struct LazyEveryOtherIterator <I: IteratorProtocol>: IteratorProtocol { var base: I mutating func next() -> I.Element? { if let _ = base.next() { return base.next() } else { return nil } } }
83. NEW TORTILLAS extension LazySequenceProtocol { func everyOther() -> LazyEveryOtherSequence<Self> { return LazyEveryOtherSequence(base: self) } }
84. PHOTOS let skipped = photos.everyOther()
85. PHOTOS Product wants every 4th photo http://www.giftofcuriosity.com/wp-content/uploads/2016/04/Letter-hopscotch-4.jpg
86. PHOTOS let skipped = photos.everyOther().everyOther()
87. RECAP
88. RECAP Collections hold our photo metadata and photos
89. RECAP Collections hold our photo metadata and photos LazyCollection's map transforms our data
90. RECAP Collections hold our photo metadata and photos LazyCollection's map transforms our data AnyCollection gives maintainable type signatures
91. RECAP Collections hold our photo metadata and photos LazyCollection's map transforms our data AnyCollection gives maintainable type signatures We can create new operators that compose
92. FINALLY Our app works!
93. P.S. IBM Swift Sandbox is legit Check out Unary indexed collections in appendix
94. GROK IT? By / Slide Deck: Brandon Kase @bkase_ https://is.gd/mEvKbt
95. APPENDIX
96. EAGER Eager (or strict) is the opposite of lazy Computed always
97. EAGER class A { var eagerField: Int = 1 + 1 /* ... */ }
98. LAZY class A { lazy var lazyField: Int = 1 + 1 /* ... */ }
99. EAGER GROUPS OF THINGS
100. EAGER GROUPS OF THINGS Eager means operations on the groups of things are eager The groups of things themselves can be strict or lazy
101. UNARY
102. UNARY let u: Unary</*...*/> u[""] // 1st element u["x"] // 2st element u["xx"] // 3st element u["xxx"] // 4st element
103. UNARY Given any input collection addressable via Ints, Create a collection addressable via unary numbers!
104. UNARY More powerful than overriding the subscript operator!
105. UNARY print(u.lazy.map{ x in x + 1 }["xxx"])
106. UNARY Let's build it
107. UNARY struct Unary<C: Collection>: Collection where C.Index == Int, C.IndexDistance == Int {
108. UNARY { var startIndex: String { /*...*/ } var endIndex: String { /*...*/ } func index(after i: String) -> String { /*...*/ } subscript(i: String) -> C.Iterator.Element { /*...*/ } }
109. UNARY private let xs: C init(_ xs: C) { self.xs = xs }
110. UNARY var startIndex: String { return "" }
111. UNARY /// O(n) endIndex var endIndex: String { return String((0...xs.count).map{ _ in "x"}) }
112. UNARY func index(after i: String) -> String { return i + "x" }
113. UNARY subscript(i: String) -> C.Iterator.Element { return xs[i.characters.count] }
114. UNARY let u: Unary<Array<Int>> = Unary([1, 2, 3, 4, 5, 6]) print(u.lazy.map{ x in x + 1 }["xxx"])
115. COLLECTION-INDEX VARIANTS How can you move to the next index? Collection (ForwardDirection) Go forward in O(1) BidirectionalCollection Go forward or backward in O(1) RandomAccessCollection Get the difference between two indices in O(1)
116. TYPE ERASURE
117. TYPE ERASURE Type erasure works by moving the type info to the constructor
118. TYPE ERASURE // one of the inits for AnySequence init<S : Sequence where S.Iterator.Element == Element, S.SubSequence : Sequence, S.SubSequence.Iterator.Element == Element, S.SubSequence.SubSequence == S.SubSequence>(_ base: S)