//! Simplest sync [Monad].
//!
//! For async, see [`super::future`].

use crate::func::*;

#[derive(SharedFunctor, CovariantFunctor)]
pub struct SoloInstance;

impl WeakFunctor for SoloInstance {
    type F<'a, A: 'a> = A;
}

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

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

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

impl<'a> Pure<'a> for SoloInstance {
    fn pure<A: 'a>(a: A) -> Self::F<'a, A> {
        a
    }
}

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

impl<'a> ApplicativeLA2<'a> for SoloInstance {
    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> {
        f(fa, fb)
    }
}

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

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

impl<'a> Applicative<'a> for SoloInstance {
    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 SoloInstance {
    fn bind<A: 'a, B: 'a>(
        fa: Self::F<'a, A>,
        f: impl 'a + FnOnce(A) -> Self::F<'a, B>,
    ) -> Self::F<'a, B> {
        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 b,
            }
        }
    }

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

impl LocalFunctor for SoloInstance {
    fn unstuff<'a, A: 'a, B: 'a>(
        state: Self::F<'a, ControlFlow<B, A>>,
    ) -> ControlFlow<Self::F<'a, B>, A>
    where
        Self: 'a,
    {
        state
    }

    fn stuff<'a, A: 'a, T: Pure<'a>>(fa: Self::F<'a, T::F<'a, A>>) -> T::F<'a, Self::F<'a, A>>
    where
        Self: 'a,
    {
        fa
    }
}

impl<'a> Fail<'a, std::convert::Infallible> for SoloInstance {
    fn fail<A: 'a>(e: std::convert::Infallible) -> Self::F<'a, A> {
        match e {}
    }
}