Answering the Existential Question

Given at FrenchKit 2018

Transcript

1. Answering the Existential Question
   Samuel Giddins
   1
   

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2. @segiddins
   Mobile Developer Experience @ Square
   Trill Core Team
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3. ⚠
   Programming Language Nerdery Ahead
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4. back to the beginning of time
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5. Objective-C
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6. NSArray *strings = @[
   @"1",
   @2.string,
   ];
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7. id strings = [NSArray arrayWithObjects:
   [NSString stringWithUTF8String:"1"],
   [[NSNumber numberWithInteger:2] stringValue],
   nil
   ];
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8. Objective-C
   ⇢Based on SmallTalk
   ⇢All message-passaging
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9. Message Passing
   receiver (self) determines what to do with each
   message
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10. sound familiar?
    we still have that in today's Objective-C -- id
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11. id is really cool
    every objc object is an id,
    and you can send any message to id
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12. even better
    you can recover class/type info from an id,
    get info like respondsToSelector:
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13. You can ask an id what it is, and it will tell you.
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14. I think, therefore I am.
    — Descartes
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15. And there you have an existential!
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16. Existential:
    something that knows what it is
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17. Simple Sounding,
    Powerful Construct
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18. existentials' power:
    ⇢Dynamic Casting
    ⇢print()
    ⇢Protocol Types
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19. When Swift was first announced...
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20. When Swift was first announced
    I worried that Ints and Bools having type information
    would make the language slow
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21. ... would make the language slow
    like Ruby
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22. In Ruby (like Objective-C),
    every object has an isa pointer
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23. struct objc_object {
    Class isa,
    uint_64t flags,
    // ivars...
    }
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24. That'd be a lot of overhead for every Int!
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25. So how does this work?
    let i: Int = 5
    let a: Any = i as Any
    if let really_i = a as? Int {
    print("this works! i is \(really_i)")
    }
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26. It doesn't work in a language like C,
    since an Int is just 32 bits
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27. !
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28. !
    Swift is smarter than C
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29. Let's step through how these 5 lines of code work
    let i: Int = 5
    let a: Any = i as Any
    if let really_i = a as? Int {
    print("this works! i is \(really_i)")
    }
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30. This is equivalent to C:
    we just store the raw integer
    let i: Int = 5
    let a: Any = i as Any
    if let really_i = a as? Int {
    print("this works! i is \(really_i)")
    }
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31. This is just like doing an
    isKindOfClass: check in ObjC --
    at runtime, we check the type metadata for a
    let i: Int = 5
    let a: Any = i as Any
    if let really_i = a as? Int {
    print("this works! i is \(really_i)")
    }
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32. And now really_i is back to being a normal Int --
    and can be used with zero overhead
    let i: Int = 5
    let a: Any = i as Any
    if let really_i = a as? Int {
    print("this works! i is \(really_i)")
    }
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33. and that's it!
    !
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34. Oh, I missed this line?
    let i: Int = 5
    let a: Any = i as Any
    if let really_i = a as? Int {
    print("this works! i is \(really_i)")
    }
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35. It turns out, this is the key to understanding
    how existentials work
    let a: Any = 5 as Any
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36. let a: Any = 5 as Any
    ⇢Compiler sees a cast from Int Any
    ⇢ Int is a non-existential type, Any is existential
    ⇢ Compiler generates a "promotion" for 5, and
    stores it as a separate instance
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37. let a: Any = 5 as Any
    ⇢A new ExistentialBox is allocated
    ⇢ Points to type metadata
    ⇢ Which also points to "thunks" for protocol
    witnesses
    ⇢ Points to (or copies) the underlying value
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38. All code referring to an Any knows it needs to indirect
    through the Box
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39. And any code that needs type metadata
    for a non-existential type
    Goes through the promotion process
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40. Trill
    Wannabe Swift, written in Swift
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41. type Foo {
    let bar: Int
    let baz: Bool
    }
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42. func foo(_ t: Any) -> Int {
    printf("[%s] entered\n", #function)
    if t is Int {
    printf("t is an Int!!\n")
    }
    return t as Int
    }
    func duplicate(_ t: Any) -> Any {
    return t as Int * 2
    }
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43. func main() {
    printf("[%s] entered\n", #function)
    let f = foo(1)
    var v: Any = 1
    if !(v is Bool) {
    printf("v: %d\n", v as Int)
    }
    v = true
    if v is Bool {
    printf("v: %s\n", v as Bool ? "true" : "false")
    }
    v = Foo(bar: 20, baz: true)
    if v is Foo {
    let foo = v as Foo
    printf("v: Foo(bar: %d, baz: %s)\n", foo.bar, foo.baz ? "true" : "false")
    }
    printf("[%s] foo returned\n", #function)
    printf("%d\n", f)
    printf("%d\n", duplicate(2) as Int)
    }
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44. /// TRILL_ANY is a special type understood by the Trill compiler as the
    /// representation of an Any value.
    typedef struct TRILL_ANY {
    void * _Nonnull _any;
    #ifdef __cplusplus
    inline AnyBox *_Nonnull any() {
    trill_assert(_any != nullptr && "passed a null value for Any");
    return reinterpret_cast(_any);
    }
    inline AnyBox *_Nonnull operator->() noexcept { return any(); }
    inline operator AnyBox*_Nonnull() { return any(); }
    #endif
    } TRILL_ANY;
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45. /// An AnyBox is a heap-allocated box that contains:
    /// - A pointer to the type metadata for an underlying value
    /// - A variably-sized payload
    struct AnyBox {
    const TypeMetadata *typeMetadata;
    static AnyBox *create(const TypeMetadata *metadata);
    AnyBox *copy();
    void updateField(uint64_t fieldNum, AnyBox *newValue);
    void *value() {
    return reinterpret_cast(
    reinterpret_cast(this) + sizeof(AnyBox));
    }
    void *fieldValuePtr(uint64_t fieldNum);
    AnyBox *extractField(uint64_t fieldNum);
    const FieldMetadata *fieldMetadata(uint64_t fieldNum);
    bool isNil();
    };
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46. func codegenPromoteToAny(value: IRValue, type: DataType) -> IRValue {
    if case .any = type {
    if storage(for: type) == .reference {
    // If we're promoting an existing Any value of a reference type, just
    // thread it through.
    return value
    } else {
    // If we're promoting an existing Any value of a value type,
    // then this should just be a copy of the existing value.
    return codegenCopyAny(value: value)
    }
    }
    let allocateAny = codegenIntrinsic(named: "trill_allocateAny")
    let meta = codegenTypeMetadata(type)
    let castMeta = builder.buildBitCast(meta, type: PointerType.toVoid, name: "mc")
    let res = builder.buildCall(allocateAny, args: [castMeta], name: "aa")
    let valPtr = codegenAnyValuePtr(res, type: type)
    builder.buildStore(value, to: valPtr)
    return res
    }
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47. What's next?
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48. What's next for Swift?
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49. Opaque Return Types
    func importantDates() ->
    opaque Collection where _.Element == Date
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50. Type Erasure
    let ints = AnyCollection = [1, 2, 3]
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51. Generalized Existentials
    let strings: Any =
    ["a", "b", "c"]
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52. Samuel Giddins
    @segiddins
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