132 lines
3.4 KiB
Rust
132 lines
3.4 KiB
Rust
//! Helper [Monad]s to defer execution until necessary.
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//! Wrapped value is just a box pointing to the constructor function.
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//!
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//! Due to semantical laziness,
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//! [`LazyInstance::replace`] and [`LazyInstance::discard_first`]/[`LazyInstance::discard_second`]
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//! actually fully cancel the "unnecessary" computation.
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//!
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//! For stackless execution see [`super::stackless`].
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use std::{cell::RefCell, rc::Rc};
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use crate::func::*;
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#[derive(CovariantFunctor)]
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pub struct LazyInstance;
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impl WeakFunctor for LazyInstance {
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type F<'a, A: 'a> = Box<dyn 'a + FnOnce() -> A>;
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}
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impl Functor for LazyInstance {
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fn fmap<'a, A: 'a, B: 'a>(f: impl 'a + FnOnce(A) -> B, fa: Self::F<'a, A>) -> Self::F<'a, B> {
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Box::new(|| f(fa()))
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}
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fn replace<'a, A: 'a, B: 'a>(fa: Self::F<'a, A>, b: B) -> Self::F<'a, B> {
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drop(fa);
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Box::new(|| b)
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}
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fn void<'a, A: 'a>(fa: Self::F<'a, A>) -> Self::F<'a, ()> {
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drop(fa);
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Box::new(|| ())
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}
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}
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impl<'a> Pure<'a> for LazyInstance {
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fn pure<A: 'a>(a: A) -> Self::F<'a, A> {
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Box::new(|| a)
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}
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}
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impl<'a> ApplicativeSeq<'a> for LazyInstance {
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fn seq<A: 'a, B: 'a>(
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ff: Self::F<'a, impl 'a + FnOnce(A) -> B>,
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fa: Self::F<'a, A>,
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) -> Self::F<'a, B> {
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Box::new(|| ff()(fa()))
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}
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}
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impl<'a> ApplicativeLA2<'a> for LazyInstance {
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fn la2<A: 'a, B: 'a, C: 'a>(
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f: impl 'a + FnOnce(A, B) -> C,
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fa: Self::F<'a, A>,
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fb: Self::F<'a, B>,
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) -> Self::F<'a, C> {
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Box::new(|| f(fa(), fb()))
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}
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}
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impl<'a> ApplicativeTuple<'a> for LazyInstance {
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fn tuple<A: 'a, B: 'a>((fa, fb): (Self::F<'a, A>, Self::F<'a, B>)) -> Self::F<'a, (A, B)> {
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Box::new(|| (fa(), fb()))
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}
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}
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impl<'a> ApplicativeSelect<'a> for LazyInstance {}
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impl<'a> Applicative<'a> for LazyInstance {
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fn discard_first<A: 'a, B: 'a>(fa: Self::F<'a, A>, fb: Self::F<'a, B>) -> Self::F<'a, B> {
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drop(fa);
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fb
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}
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fn discard_second<A: 'a, B: 'a>(fa: Self::F<'a, A>, fb: Self::F<'a, B>) -> Self::F<'a, A> {
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drop(fb);
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fa
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}
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}
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impl<'a> Monad<'a> for LazyInstance {
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fn bind<A: 'a, B: 'a>(
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fa: Self::F<'a, A>,
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f: impl 'a + FnOnce(A) -> Self::F<'a, B>,
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) -> Self::F<'a, B> {
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Box::new(|| f(fa())())
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}
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fn iterate<B: 'a>(mut f: impl Iterative<'a, T = Self, B = B>) -> Self::F<'a, B> {
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loop {
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match f.next()() {
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ControlFlow::Continue(next_f) => f = next_f,
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ControlFlow::Break(b) => return Self::pure(b),
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}
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}
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}
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fn join<A: 'a>(ffa: Self::F<'a, Self::F<'a, A>>) -> Self::F<'a, A> {
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Box::new(|| ffa()())
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}
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}
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fn unshare<'a, A: 'a + Clone>(shared: &mut Option<Box<dyn 'a + FnOnce() -> A>>) -> A {
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let a = shared.take().expect(
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"cannot evaluate a missing shared lazy value. probably, the shared value depends on itself",
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)();
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let cloned = a.clone();
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*shared = Some(Box::new(|| cloned));
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a
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}
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impl SharedFunctor for LazyInstance {
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type Shared<'a, A: 'a + Clone> = Rc<RefCell<Option<Box<dyn 'a + FnOnce() -> A>>>>
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where
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Self: 'a;
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fn share<'a, A: 'a + Clone>(fa: Self::F<'a, A>) -> Self::Shared<'a, A>
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where
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Self: 'a,
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{
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Rc::new(RefCell::new(Some(fa)))
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}
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fn unshare<'a, A: 'a + Clone>(sa: Self::Shared<'a, A>) -> Self::F<'a, A>
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where
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Self: 'a,
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{
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Box::new(move || unshare(&mut *sa.borrow_mut()))
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}
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}
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