What is a nominal type?

The slide for the talk on:
try! Swift Tokyo 2017 RejectCon - connpass
https://rmp-quipper.connpass.com/event/49316/

Swiftで、Anyなどの一部の型にextensionを書こうとしたときのエラー文言「Non-nominal type 'Any' cannot be extended」
「Non-nominal type」とは何なのか？　について掘っていたら、型システムの基本的な分類に辿り着きましたよ、というお話です。
英語および日本語の資料です。

Transcript

1. What is a nominal type? by takasek 2017/2/28 try! Swift RejectCon 1
2. I ❤ extension 2
3. I ❤ to extend everything 3
5. Int says hello!! 5
6. Everyone, say hello!! 6
8. UIViewControllerʮ!ʯ NSStringʮ!ʯ NSErrorʮ!ʯ 8
9. Anyone else?! 9
10. Any-san kochira! 10
12. !" 12
13. How about you guys, closure-san?! tuple-san?! 13
15. !" 15
16. ! 16
17. ❓❔❓❔❓❔❓❔❓ Non-nominal type ❔❓❔❓❔❓❔❓❔ 17
18. The Swift Programming Language (Swift 3.0.1): Types https://developer.apple.com/library/content/documentation/Swift/ Conceptual/SwiftProgrammingLanguage/Types.html " In Swift, there are two kinds of types: named types and compound types. " 18
19. Named type == Nominal type ? Compound type == Non-nominal type ? 19
22. ܕγεςϜೖ໳ −ϓϩάϥϛϯάݴޠͱܕͷཧ࿦− Types and Programming Languages (MIT Press) 4 JavaͷΑ͏ʹɺ໊લ͕ॏཁͰɺ෦෼ܕؔ܎͕໌ࣔతʹએݴ͞ΕΔܕ γεςϜΛ໊લతͱ͍͏ɻ (…)໊લ͕ຊ࣭తͰ͸ͳ͘ɺ෦෼ܕؔ܎͕ܕͷߏ଄ͷ্ʹ௚઀ఆٛ͞ ΕΔܕγεςϜΛߏ଄తͱ͍͏ɻ (P197) 4 Type systems like Java's, in which names are significant and subtyping is explicitly declared, are alled nominal. Type systems (...) in which names are inessential and subtyping is defined directly on the structures of types are called structural. (P252) 22
23. Named type == Nominal type ! Compound type == Non-nominal type == Structural type ! 23
24. a named type / a compound type 4 Named types: 4 classes 4 structures 4 enumerations 4 protocols 4 Compound types: 4 function types 4 tuple types 24
25. 4 Compound (structural) types: 4 function types 4 tuple types !❗ɹɹɹɹɹɹɹɹ※ Any ≒ empty set of protocols 25
26. Structural type vs Nominal type 26
27. structs as nominal types struct ItemA { let id: Int let name: String } struct ItemB { let id: Int let name: String } let array: [ItemA] = [ ItemA(id: 1, name: "!"), ItemB(id: 2, name: """) as ItemA //# ] 27
28. tuples as structural types typealias ItemA = ( id: Int, name: String ) typealias ItemB = ( id: Int, name: String ) let array: [ItemA] = [ ItemA(id: 1, name: "!"), ItemB(id: 2, name: """) //# ] 28
29. Nominal type vs Structural type 4 structural type 4 ⭕ flexiblity 4 permits the creation of ad hoc types and protocols. 4 https://en.wikipedia.org/wiki/Structural_type_system 4 nominal type 4 ⭕ better type-safety 4 useful not only during typechecking, but at run time as well. (...) e.g., Java's instanceOf test and downcasting operation. 4 Types and Programming Languages P252 29
30. ✨better type-safety✨ 30
31. Unsafety of structural types func concat(number: Int, unit: String) { print("\(number)\(unit)") } 31
32. Unsafety of structural types func concat(number: Int, unit: String) { print("\(number)\(unit)") } typealias Item = (id: Int, name: String) let onigiri = Item(id: 1, name: "!") 32
33. Unsafety of structural types func concat(number: Int, unit: String) { print("\(number)\(unit)") } typealias Item = (id: Int, name: String) let onigiri = Item(id: 1, name: "!") concat(onigiri) // ⛔error! 33
34. Unsafety of structural types func concat(number: Int, unit: String) { print("\(number)\(unit)") } typealias Item = (id: Int, name: String) let onigiri = Item(id: 1, name: "!") let f: ((Int, String)) -> Void = concat 34
35. Unsafety of structural types func concat(number: Int, unit: String) { print("\(number)\(unit)") } typealias Item = (id: Int, name: String) let onigiri = Item(id: 1, name: "!") let f: ((Int, String)) -> Void = concat f(onigiri) // 1! 35
36. (number: Int, unit: String) == (id: Int, name: String) 36
37. ! 37
38. Why Non-nominal type cannot be extended? Because... Nominal typing is useful at preventing "accidental" type equivalence! Nominal type system - Wikipedia https://en.wikipedia.org/wiki/Nominal_type_system 38
39. The Swift Programming Language (Swift 3.0.1): The Basics https://developer.apple.com/library/content/documentation/Swift/Conceptual/ SwiftProgrammingLanguage/TheBasics.html " Swift is a type-safe language. ɹA type safe language encourages you ɹto be clear about the types of values ɹyour code can work with. " 39
40. I ❤ Swift ! 40
41. ༨ஊ digression 41
42. Swift effectively uses structural types' flexibility under the core. 42