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

//! Implementation of [`Monad`] for [`Option<A>`].
//!
//! If any of the input values are [`None`], you can expect the output to be [`None`] as well.
//! That includes
//! [`OptionInstance::replace`] and [`OptionInstance::discard_first`]/[`OptionInstance::discard_second`],
//! even if the value of the option would be ignored.
//!
//! For [`Result<A, E>`] alternative see [`super::result`]

use crate::func::*;

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

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

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

impl ApplicativeSeq for OptionInstance {
    fn seq<'a, 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 ApplicativeLA2 for OptionInstance {
    fn la2<'a, 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> ApplicativeTuple<'a> for OptionInstance {
    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> ApplicativeSelect<'a> for OptionInstance {}

impl<'a> Applicative<'a> for OptionInstance {
    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> Monad<'a> for OptionInstance {
    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 LocalFunctor for OptionInstance {
    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 {
            Some(ControlFlow::Continue(a)) => ControlFlow::Continue(a),
            Some(ControlFlow::Break(b)) => ControlFlow::Break(Some(b)),
            None => ControlFlow::Break(None),
        }
    }

    fn stuff<'a, A: 'a, T: 'a + Pure>(fa: Self::F<'a, T::F<'a, A>>) -> T::F<'a, Self::F<'a, A>>
    where
        Self: 'a,
    {
        match fa {
            Some(ua) => T::fmap(Some, ua),
            None => T::pure(None),
        }
    }
}

impl<'a> Fail<'a, ()> for OptionInstance {
    fn fail<A: 'a>(_e: ()) -> Self::F<'a, A> {
        None
    }
}

#[cfg(test)]
mod option_tests {
    use super::{test_suite, tests, Functor};

    use super::OptionInstance as T;

    impl tests::Eqr for T {
        fn eqr<'a, A: PartialEq + std::fmt::Debug + 'a>(
            name: &'a str,
            left: Self::F<'a, A>,
            right: Self::F<'a, A>,
        ) -> tests::R {
            tests::eqr(name, left, right)
        }
    }

    impl test_suite::FunctorTestSuite for T {
        fn sample<'a, A: 'a, F: FnMut(&'a dyn Fn(A) -> Self::F<'a, A>)>(mut f: F)
        where
            Self::F<'a, A>: 'a,
        {
            f(&|_| None);
            f(&|a| Some(a));
        }
    }

    #[test]
    fn fmap_f_none_is_none() {
        assert_eq!(T::fmap(|_: ()| (), None), None);
    }

    #[test]
    fn fmap_f_some_a_is_some_f_a() {
        assert_eq!(T::fmap(|x| x * x, Some(2)), Some(4));
    }

    #[test]
    fn replace_none_b_is_none() {
        assert_eq!(T::replace(None::<i32>, 1), None);
        assert_eq!(T::void(None::<i32>), None);
    }

    #[test]
    fn replace_some_a_b_is_some_b() {
        assert_eq!(T::replace(Some(1), 2), Some(2));
        assert_eq!(T::void(Some(1)), Some(()));
    }

    #[test]
    fn monad_follows_laws() {
        test_suite::monad_follows_laws::<T>().unwrap();
    }
}