haar_lib/algebra/
first_last.rs

1//! First, Lastモノイド
2pub use crate::algebra::traits::*;
3use crate::impl_algebra;
4
5/// 最初に出現する`Some`を返す演算。
6#[derive(Clone, Copy, Default, Debug, PartialEq, Eq)]
7pub struct First<T>(pub Option<T>);
8/// 最後に出現する`Some`を返す演算。
9#[derive(Clone, Copy, Default, Debug, PartialEq, Eq)]
10pub struct Last<T>(pub Option<T>);
11
12impl_algebra!(
13    [T]; First<T>;
14    op: |a: Self, b| match a.0 {
15        Some(_) => a,
16        None => b
17    };
18    id: Self(None);
19    assoc;
20    idem;
21);
22impl_algebra!(
23    [T]; Last<T>;
24    op: |a, b: Self| match b.0 {
25        Some(_) => b,
26        None => a
27    };
28    id: Self(None);
29    assoc;
30    idem;
31);
32
33#[cfg(test)]
34mod tests {
35    use super::*;
36
37    #[test]
38    fn test() {
39        let a = [None, None, Some(1), None, Some(3), Some(5)];
40
41        let b: Vec<_> = a.into_iter().map(First).collect();
42        dbg!(b.iter().fold(First::id(), |x, y| First::op(x, *y)));
43
44        let b: Vec<_> = a.into_iter().map(Last).collect();
45        dbg!(b.iter().fold(Last::id(), |x, y| Last::op(x, *y)));
46    }
47}
haar_lib/algebra/first_last.rs

haar_lib/algebra/
first_last.rs
