radn-rs/src/func/instances/result.rs

//! Implementation of [`MonadFail<E>`] for [`Result<A, E>`].
//!
//! If any of the input values are [`Err`], you can expect the output to be [`Err`] as well.
//! That includes
//! [`ResultInstance::replace`] and [`ResultInstance::discard_first`]/[`ResultInstance::discard_second`],
//! even if the value of the option would be ignored.
//!
//! For [`Option<A>`] alternative see [`super::option`]

use crate::func::*;

#[derive(SharedFunctor, CovariantFunctor)]
pub struct ResultInstance<E>(E);

impl<E> WeakFunctor for ResultInstance<E> {
    type F<'a, A: 'a> = Result<A, E> where Self: 'a;
}

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

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

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

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

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

impl<'a, E: 'a> ApplicativeLA2<'a> for ResultInstance<E> {
    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> {
        Self::pure(f(fa?, fb?))
    }
}

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

impl<'a, E: 'a> ApplicativeSelect<'a> for ResultInstance<E> {}

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

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

impl<'a, E: 'a> Monad<'a> for ResultInstance<E> {
    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 Self::pure(b),
            }
        }
    }

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

impl<E> LocalFunctor for ResultInstance<E> {
    fn unstuff<'a, A: 'a, B: 'a>(
        state: Self::F<'a, ControlFlow<B, A>>,
    ) -> ControlFlow<Self::F<'a, B>, A>
    where
        Self: 'a,
    {
        match state {
            Ok(ControlFlow::Continue(a)) => ControlFlow::Continue(a),
            Ok(ControlFlow::Break(b)) => ControlFlow::Break(Ok(b)),
            Err(e) => ControlFlow::Break(Err(e)),
        }
    }

    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,
    {
        match fa {
            Ok(ua) => T::fmap(Ok, ua),
            Err(e) => T::pure(Err(e)),
        }
    }
}

impl<'a, E: 'a> Fail<'a, E> for ResultInstance<E> {
    fn fail<A: 'a>(e: E) -> Self::F<'a, A> {
        Err(e)
    }
}

pub struct ResultFailAny;

trait ResultExt<'a, A: 'a, E0: 'a>: 'a {
    fn bind_err<E1: 'a>(self, f: impl 'a + FnOnce(E0) -> Result<A, E1>) -> Result<A, E1>;
}

impl<'a, A: 'a, E0: 'a> ResultExt<'a, A, E0> for Result<A, E0> {
    fn bind_err<E1: 'a>(self, f: impl 'a + FnOnce(E0) -> Result<A, E1>) -> Result<A, E1> {
        match self {
            Ok(a) => Ok(a),
            Err(e) => f(e),
        }
    }
}

impl<'a> MonadFailAny<'a> for ResultFailAny {
    type W<E: 'a> = ResultInstance<E>;

    type T = instances::solo::SoloInstance;

    fn unstuff<A: 'a, E: 'a>(wa: WrapE<'a, A, E, Self>) -> Wrap<'a, Result<A, E>, Self::T> {
        wa
    }

    fn stuff<A: 'a, E: 'a>(fa: Wrap<'a, Result<A, E>, Self::T>) -> WrapE<'a, A, E, Self> {
        fa
    }

    fn map_err<A: 'a, E0: 'a, E1: 'a>(
        wa: WrapE<'a, A, E0, Self>,
        f: impl 'a + FnOnce(E0) -> E1,
    ) -> WrapE<'a, A, E1, Self> {
        wa.map_err(f)
    }

    fn bind_err<A: 'a, E0: 'a, E1: 'a>(
        wa: WrapE<'a, A, E0, Self>,
        f: impl 'a + FnOnce(E0) -> WrapE<'a, A, E1, Self>,
    ) -> WrapE<'a, A, E1, Self> {
        wa.bind_err(f)
    }

    fn bind<A: 'a, B: 'a, E0: 'a, E1: 'a>(
        wa: WrapE<'a, A, E0, Self>,
        f: impl 'a + FnOnce(Result<A, E0>) -> WrapE<'a, B, E1, Self>,
    ) -> WrapE<'a, B, E1, Self> {
        f(wa)
    }

    fn rotate_out<A: 'a, E0: 'a, E1: 'a>(
        wa: WrapE<'a, Result<A, E1>, E0, Self>,
    ) -> WrapE<'a, A, Result<E1, E0>, Self> {
        match wa {
            Ok(Ok(a)) => Ok(a),
            Ok(Err(e)) => Err(Ok(e)),
            Err(e) => Err(Err(e)),
        }
    }
}

pub struct ResultFailOver<T>(T);

impl<'a, T: Monad<'a>> MonadFailAny<'a> for ResultFailOver<T> {
    type W<E: 'a> = super::composition::CompositionInstance<T, ResultInstance<E>>;

    type T = T;

    fn unstuff<A: 'a, E: 'a>(wa: WrapE<'a, A, E, Self>) -> Wrap<'a, Result<A, E>, Self::T> {
        wa
    }

    fn stuff<A: 'a, E: 'a>(fa: Wrap<'a, Result<A, E>, Self::T>) -> WrapE<'a, A, E, Self> {
        fa
    }

    fn map_err<A: 'a, E0: 'a, E1: 'a>(
        wa: WrapE<'a, A, E0, Self>,
        f: impl 'a + FnOnce(E0) -> E1,
    ) -> WrapE<'a, A, E1, Self> {
        T::fmap(|a| a.map_err(f), wa)
    }

    fn bind_err<A: 'a, E0: 'a, E1: 'a>(
        wa: WrapE<'a, A, E0, Self>,
        f: impl 'a + FnOnce(E0) -> WrapE<'a, A, E1, Self>,
    ) -> WrapE<'a, A, E1, Self> {
        T::bind(wa, |a| match a {
            Ok(a) => T::pure(Ok(a)),
            Err(e) => f(e),
        })
    }

    fn bind<A: 'a, B: 'a, E0: 'a, E1: 'a>(
        wa: WrapE<'a, A, E0, Self>,
        f: impl 'a + FnOnce(Result<A, E0>) -> WrapE<'a, B, E1, Self>,
    ) -> WrapE<'a, B, E1, Self> {
        T::bind(wa, f)
    }

    fn rotate_out<A: 'a, E0: 'a, E1: 'a>(
        wa: WrapE<'a, Result<A, E1>, E0, Self>,
    ) -> WrapE<'a, A, Result<E1, E0>, Self> {
        T::fmap(<ResultFailAny as MonadFailAny>::rotate_out, wa)
    }
}