//! 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 [`stackless`].
//!
//! [`stackless`]: super::stackless

use crate::func::class_prelude::*;

pub struct LazyInstance;

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

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

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

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

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

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

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

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

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

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

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

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

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

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