//! Helper [Monad]s to defer execution until necessary.
//! Wrapped value is just a box pointing to the constructor function.
//!
//! Due to semantical laziness,
//! [`LazyInstance::replace`] and [`LazyInstance::discard_first`]/[`LazyInstance::discard_second`]
//! actually fully cancel the "unnecessary" computation.
//!
//! For stackless execution see [`super::stackless`].

use std::{cell::RefCell, rc::Rc};

use crate::func::*;

#[derive(CovariantFunctor)]
pub struct LazyInstance;

impl WeakFunctor for LazyInstance {
    type F<'a, A: 'a> = Box<dyn 'a + FnOnce() -> A>;
}

impl Functor for LazyInstance {
    fn fmap<'a, A: 'a, B: 'a>(f: impl 'a + FnOnce(A) -> B, fa: Self::F<'a, A>) -> Self::F<'a, B> {
        Box::new(|| f(fa()))
    }

    fn replace<'a, A: 'a, B: 'a>(fa: Self::F<'a, A>, b: B) -> Self::F<'a, B> {
        drop(fa);
        Box::new(|| b)
    }

    fn void<'a, A: 'a>(fa: Self::F<'a, A>) -> Self::F<'a, ()> {
        drop(fa);
        Box::new(|| ())
    }
}

impl<'a> Pure<'a> for LazyInstance {
    fn pure<A: 'a>(a: A) -> Self::F<'a, A> {
        Box::new(|| a)
    }
}

impl<'a> ApplicativeSeq<'a> for LazyInstance {
    fn seq<A: 'a, B: 'a>(
        ff: Self::F<'a, impl 'a + FnOnce(A) -> B>,
        fa: Self::F<'a, A>,
    ) -> Self::F<'a, B> {
        Box::new(|| ff()(fa()))
    }
}

impl<'a> ApplicativeLA2<'a> for LazyInstance {
    fn la2<A: 'a, B: 'a, C: 'a>(
        f: impl 'a + FnOnce(A, B) -> C,
        fa: Self::F<'a, A>,
        fb: Self::F<'a, B>,
    ) -> Self::F<'a, C> {
        Box::new(|| f(fa(), fb()))
    }
}

impl<'a> ApplicativeTuple<'a> for LazyInstance {
    fn tuple<A: 'a, B: 'a>((fa, fb): (Self::F<'a, A>, Self::F<'a, B>)) -> Self::F<'a, (A, B)> {
        Box::new(|| (fa(), fb()))
    }
}

impl<'a> ApplicativeSelect<'a> for LazyInstance {}

impl<'a> Applicative<'a> for LazyInstance {
    fn discard_first<A: 'a, B: 'a>(fa: Self::F<'a, A>, fb: Self::F<'a, B>) -> Self::F<'a, B> {
        drop(fa);
        fb
    }

    fn discard_second<A: 'a, B: 'a>(fa: Self::F<'a, A>, fb: Self::F<'a, B>) -> Self::F<'a, A> {
        drop(fb);
        fa
    }
}

impl<'a> Monad<'a> for LazyInstance {
    fn bind<A: 'a, B: 'a>(
        fa: Self::F<'a, A>,
        f: impl 'a + FnOnce(A) -> Self::F<'a, B>,
    ) -> Self::F<'a, B> {
        Box::new(|| f(fa())())
    }

    fn iterate<B: 'a>(mut f: impl Iterative<'a, T = Self, B = B>) -> Self::F<'a, B> {
        loop {
            match f.next()() {
                ControlFlow::Continue(next_f) => f = next_f,
                ControlFlow::Break(b) => return Self::pure(b),
            }
        }
    }

    fn join<A: 'a>(ffa: Self::F<'a, Self::F<'a, A>>) -> Self::F<'a, A> {
        Box::new(|| ffa()())
    }
}

fn unshare<'a, A: 'a + Clone>(shared: &mut Option<Box<dyn 'a + FnOnce() -> A>>) -> A {
    let a = shared.take().expect(
        "cannot evaluate a missing shared lazy value. probably, the shared value depends on itself",
    )();
    let cloned = a.clone();
    *shared = Some(Box::new(|| cloned));
    a
}

impl SharedFunctor for LazyInstance {
    type Shared<'a, A: 'a + Clone> = Rc<RefCell<Option<Box<dyn 'a + FnOnce() -> A>>>>
    where
        Self: 'a;

    fn share<'a, A: 'a + Clone>(fa: Self::F<'a, A>) -> Self::Shared<'a, A>
    where
        Self: 'a,
    {
        Rc::new(RefCell::new(Some(fa)))
    }

    fn unshare<'a, A: 'a + Clone>(sa: Self::Shared<'a, A>) -> Self::F<'a, A>
    where
        Self: 'a,
    {
        Box::new(move || unshare(&mut *sa.borrow_mut()))
    }
}