Rust で遅延評価を 実装してみる

RustのLT会！ Rust入門者の集い #3

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1. Rust Ͱ஗ԆධՁΛ
   ࣮૷ͯ͠ΈΔ
   RustͷLTձʂ Rustೖ໳ऀͷू͍ #3
   @Linda_pp
   @rhysd
   
   

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2. ͋Β͢͡
   • ७ਮؔ਺ܕσʔλߏ଄ΛಡΈͳ͕
   Β Rust Ͱ࣮૷͍ͯͨ͋͠Δ೔ɼ
   ஗ԆධՁΛ࢖͏σʔλߏ଄͕ग़ݱ
   • Rust ͸஗ԆධՁ͡Όͳ͍…
   • ࣮૷ͨ͠ https://goo.gl/qMGuZQ
   
   

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3. ஗ԆධՁͱ͸
   ࣜΛ͙͢ʹ͸ධՁͤͣɼඞཁʹͳͬͨ࣌ʹධ
   Ձ͢Δ
   
   

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4. ஗Ԇ͠ͳ͍ධՁ
   1 // 1000ݸͷૉ਺Λͭ͘ΔʢΊͬͪΌॏ͍ॲཧʣ
   2 let n = primes1000();
   3
   4 // ...
   5
   6 // 100൪໨දࣔ
   7 println!("{}", vec[99]);
   8
   9 // 1000൪໨දࣔ
   10 println!("{}", vec[999]);
   
   ιʔεɿ https://goo.gl/qMGuZQ
   

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5. ஗ԆධՁ
   1 // lazily! Ͱғ·Εͨࣜ͸஗ԆධՁ͞ΕΔ
   2 // ͜ͷ࣌఺Ͱ͸ vec ͸ܭࢉ͞Ε͍ͯͳ͍
   3 let vec = lazily!{
   4 prime1000th()
   5 };
   6
   7 // vec Λ࢖༻͢ΔλΠϛϯάͰॳΊͯ vec ΛධՁ
   8 println!("{}", vec[99]);
   9
   10 // Ұ౓ධՁͨ݁͠Ռ͸Ωϟογϡ͞Εͯ࢖͍·Θ͞ΕΔ
   11 println!("{}", vec[999]);
   
   ιʔεɿ https://goo.gl/qMGuZQ
   

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6. σʔλͷ࣋ͪํ
   1 // αϯΫɿܭࢉΛද͢σʔλߏ଄
   2 enum Thunk<'a, T: 'a> {
   3 // ·ͩܭࢉ͞Εͯͳ͍ঢ়ଶΛΫϩʔδϟͰ࣋ͭ
   4 NotYet(Box T + 'a>),
   5 // ܭࢉࡁΈ
   6 Memo(T),
   7 }
   8
   9 // ஗Ԇ͞ΕͨࣜΛද͢ߏ଄ମ Delayed
   10 pub struct Delayed<'a, T: 'a> {
   11 thunk: RefCell>>
   12 }
   
   ιʔεɿ https://goo.gl/qMGuZQ
   

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7. σʔλͷ࣋ͪํ
   1 impl<'a, T: 'a> Delayed<'a, T> {
   2 // ৽͍͠஗ԆධՁࣜΛੜ੒
   3 // ΫϩʔδϟΛड͚औͬͯ RefCell ʹಥͬࠐΉ
   4 pub fn new(f: F) -> Self where F: Fn() -> T + 'a {
   5 Delayed { thunk: RefCell::new(
   6 Box::new(
   7 NotYet(Box::new(f))
   8 )
   9 ) }
   10 }
   11 }
   12
   13 // ϚΫϩͰ move Ωϟϓνϟ͢ΔΫϩʔδϟʹల։͢Δ
   14 // lazily!{42} -> Delayed::new(move || { 42 })
   15 #[macro_export]
   16 macro_rules! lazily {
   17 ($($b:tt)+) => {
   18 self::Delayed::new(move || { $($b)+ })
   19 }
   20 }
   ιʔεɿ https://goo.gl/qMGuZQ
   

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8. force: ஗Ԇ͞ΕͨࣜΛධՁ
   1 impl<'a, T: 'a> Delayed<'a, T> {
   2 pub fn force(&self) {
   3 // ͜͜Ͱ mut ʹ͠ͳ͍ͱαϯΫ͕ߋ৽Ͱ͖ͳ͍ͷͰɼ
   4 // RefCell ͰแΉ
   5 let mut thunk = &mut *self.thunk.borrow_mut();
   6 let val = match **thunk {
   7 // ະධՁͳΒอ͍࣋ͯͨ͠ΫϩʔδϟΛ࣮ߦ͠ɼ
   8 // ܭࢉΛ࣮ߦ͢Δɽ
   9 NotYet(ref invoke) => {
   10 Box::new(Memo(invoke()))
   11 },
   12 // ͢ͰʹܭࢉࡁΈͳΒԿ΋͠ͳ͍
   13 Memo(_) => return,
   14 };
   15 // ܭࢉͨ͠஋Λ͓࣋ͬͯ͘
   16 *thunk = val;
   17 }
   18 }
   ιʔεɿ https://goo.gl/qMGuZQ
   

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9. ஗ԆධՁͷλΠϛϯά
   Rc ͕΍͍ͬͯΔΑ͏ʹɼDerefτϨΠτ
   Λ࢖ͬͯɼϝιουݺͼग़͠ͷλΠϛϯάͰ
   ࣗಈͰத਎͕ධՁ͞ΕΔΑ͏ʹ͢Δ
   1 let s = lazily!("foo".to_string())
   2
   3 // s ͕ࣗಈͰධՁ͞ΕΔ
   4 s.as_str();
   5
   6 // * ԋࢉࢠͰ໌ࣔతʹධՁ΋Ͱ͖Δ
   7 *s;
   
   ιʔεɿ https://goo.gl/qMGuZQ
   

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10. Deref Ͱࢀর࣌ʹࣗಈධՁ
    1 impl<'a, T: 'a> Deref for Delayed<'a, T> {
    2 // T ͕ධՁ݁Ռͷ஋ͷܕ
    3 type Target = T;
    4 fn deref(&self) -> &T {
    5 self.force();
    6 // RefCell ͷத਎ΛҰ࣌తʹऔΓग़͢
    7 let thunk = unsafe {
    8 self.thunk.as_ptr().as_ref().unwrap()
    9 };
    10 match **thunk {
    11 // αϯΫͷத΁ͷࢀরΛฦ͢
    12 Memo(ref v) => v,
    13 _ => unreachable!(),
    14 }
    15 }
    16 }
    
    ιʔεɿ https://goo.gl/qMGuZQ
    

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11. ͭͬͨ͘஗ԆධՁͰ஗ԆϦετ
    Λͭͬͨ͘ײ૝
    • ΄΅ॻ੶಺ͷઆ໌௨Γʹίʔυ͕ॻ͚ͯྑ͍ײ͡
    • Deref ͸༧૝֎ͷλΠϛϯάͰܭࢉ͕࣮ߦ͞Εͯ͠·
    ͏͜ͱ͕͋ͬͨɽ
    • Clone Λ࣮૷͢ΔͨΊʹ݁ہ Box Λ Rc ʹ͢Δඞཁ͕
    ͋ͬͨ
    • ϜʔϒΩϟϓνϟͰण໋͕བྷΉͷͰɼlazily! Ͱੜ੒͠
    ͨ஋ͷѻ͍͕Ϝζ͍
    
    ιʔεɿ https://goo.gl/qMGuZQ
    

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