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Rust で遅延評価を 実装してみる
Linda_pp
June 27, 2017
Technology
3
830
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
Linda_pp
June 27, 2017
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Transcript
Rust ͰԆධՁΛ ࣮ͯ͠ΈΔ RustͷLTձʂ Rustೖऀͷू͍ #3 @Linda_pp @rhysd
͋Β͢͡ • ७ਮؔܕσʔλߏΛಡΈͳ͕ Β Rust Ͱ࣮͍ͯͨ͋͠Δɼ ԆධՁΛ͏σʔλߏ͕ग़ݱ • Rust ԆධՁ͡Όͳ͍…
• ࣮ͨ͠ https://goo.gl/qMGuZQ
ԆධՁͱ ࣜΛ͙͢ʹධՁͤͣɼඞཁʹͳͬͨ࣌ʹධ Ձ͢Δ
Ԇ͠ͳ͍ධՁ 1 // 1000ݸͷૉΛͭ͘ΔʢΊͬͪΌॏ͍ॲཧʣ 2 let n = primes1000(); 3
4 // ... 5 6 // 100൪දࣔ 7 println!("{}", vec[99]); 8 9 // 1000൪දࣔ 10 println!("{}", vec[999]); ιʔεɿ https://goo.gl/qMGuZQ
ԆධՁ 1 // lazily! Ͱғ·ΕͨࣜԆධՁ͞ΕΔ 2 // ͜ͷ࣌Ͱ vec ܭࢉ͞Ε͍ͯͳ͍
3 let vec = lazily!{ 4 prime1000th() 5 }; 6 7 // vec Λ༻͢ΔλΠϛϯάͰॳΊͯ vec ΛධՁ 8 println!("{}", vec[99]); 9 10 // ҰධՁͨ݁͠ՌΩϟογϡ͞Ε͍ͯ·Θ͞ΕΔ 11 println!("{}", vec[999]); ιʔεɿ https://goo.gl/qMGuZQ
σʔλͷ࣋ͪํ 1 // αϯΫɿܭࢉΛද͢σʔλߏ 2 enum Thunk<'a, T: 'a> {
3 // ·ͩܭࢉ͞Εͯͳ͍ঢ়ଶΛΫϩʔδϟͰ࣋ͭ 4 NotYet(Box<Fn() -> T + 'a>), 5 // ܭࢉࡁΈ 6 Memo(T), 7 } 8 9 // Ԇ͞ΕͨࣜΛද͢ߏମ Delayed 10 pub struct Delayed<'a, T: 'a> { 11 thunk: RefCell<Box<Thunk<'a, T>>> 12 } ιʔεɿ https://goo.gl/qMGuZQ
σʔλͷ࣋ͪํ 1 impl<'a, T: 'a> Delayed<'a, T> { 2 //
৽͍͠ԆධՁࣜΛੜ 3 // ΫϩʔδϟΛड͚औͬͯ RefCell ʹಥͬࠐΉ 4 pub fn new<F>(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
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 // ͢ͰʹܭࢉࡁΈͳΒԿ͠ͳ͍ 13 Memo(_) => return, 14 }; 15 // ܭࢉͨ͠Λ͓࣋ͬͯ͘ 16 *thunk = val; 17 } 18 } ιʔεɿ https://goo.gl/qMGuZQ
ԆධՁͷλΠϛϯά Rc<T> ͕͍ͬͯΔΑ͏ʹɼDerefτϨΠτ Λͬͯɼϝιουݺͼग़͠ͷλΠϛϯάͰ ࣗಈͰத͕ධՁ͞ΕΔΑ͏ʹ͢Δ 1 let s = lazily!("foo".to_string())
2 3 // s ͕ࣗಈͰධՁ͞ΕΔ 4 s.as_str(); 5 6 // * ԋࢉࢠͰ໌ࣔతʹධՁͰ͖Δ 7 *s; ιʔεɿ https://goo.gl/qMGuZQ
Deref Ͱࢀর࣌ʹࣗಈධՁ 1 impl<'a, T: 'a> Deref for Delayed<'a, T>
{ 2 // T ͕ධՁ݁Ռͷͷܕ 3 type Target = T; 4 fn deref(&self) -> &T { 5 self.force(); 6 // RefCell ͷதΛҰ࣌తʹऔΓग़͢ 7 let thunk = unsafe { 8 self.thunk.as_ptr().as_ref().unwrap() 9 }; 10 match **thunk { 11 // αϯΫͷதͷࢀরΛฦ͢ 12 Memo(ref v) => v, 13 _ => unreachable!(), 14 } 15 } 16 } ιʔεɿ https://goo.gl/qMGuZQ
ͭͬͨ͘ԆධՁͰԆϦετ Λͭͬͨ͘ײ • ΄΅ॻ੶ͷઆ໌௨Γʹίʔυ͕ॻ͚ͯྑ͍ײ͡ • Deref ༧֎ͷλΠϛϯάͰܭࢉ͕࣮ߦ͞Εͯ͠· ͏͜ͱ͕͋ͬͨɽ • Clone
Λ࣮͢ΔͨΊʹ݁ہ Box Λ Rc ʹ͢Δඞཁ͕ ͋ͬͨ • ϜʔϒΩϟϓνϟͰण໋͕བྷΉͷͰɼlazily! Ͱੜ͠ ͨͷѻ͍͕Ϝζ͍ ιʔεɿ https://goo.gl/qMGuZQ