partial_functional/monoid/
first.rs

1use crate::semigroup::Semigroup;
2
3/// A semigroup over Option by returning the first value available.
4/// ```
5/// # use partial_functional::prelude::*;
6/// let five = First::from(5);
7/// let no_value = First(None);
8/// let ten = First::from(10);
9/// assert_eq!(First::from(10), ten.combine(five));
10/// assert_eq!(First::from(10), ten.combine(no_value));
11/// assert_eq!(First(None), no_value.combine(no_value));
12/// ```
13#[derive(Debug, Clone, Copy, PartialEq, Eq, PartialOrd, Ord, Hash)]
14pub struct First<T>(pub Option<T>);
15
16impl<T> Default for First<T> {
17    fn default() -> Self {
18        Self(None)
19    }
20}
21
22impl<T> From<T> for First<T> {
23    fn from(value: T) -> Self {
24        Self(Some(value))
25    }
26}
27
28impl<T> From<Option<T>> for First<T> {
29    fn from(value: Option<T>) -> Self {
30        Self(value)
31    }
32}
33
34impl<T> From<First<T>> for Option<T> {
35    fn from(value: First<T>) -> Self {
36        value.0
37    }
38}
39
40impl<T> Semigroup for First<T> {
41    fn combine(self, rhs: Self) -> Self {
42        Self(self.0.or(rhs.0))
43    }
44}
45
46#[cfg(test)]
47mod tests {
48    use crate::monoid::Monoid;
49
50    use super::*;
51
52    use quickcheck::Arbitrary;
53    use quickcheck_macros::quickcheck;
54
55    impl Arbitrary for First<u32> {
56        fn arbitrary(g: &mut quickcheck::Gen) -> Self {
57            First(Option::<u32>::arbitrary(g))
58        }
59
60        fn shrink(&self) -> Box<dyn Iterator<Item = Self>> {
61            Box::new(Option::<u32>::shrink(&self.0).map(|x| x.into()))
62        }
63    }
64
65    #[quickcheck]
66    fn identity_property() {
67        assert_eq!(First::<u32>(None), First::empty())
68    }
69
70    #[quickcheck]
71    fn first_is_set_to_the_first_non_none_value_in_a_list(vec: Vec<First<u32>>) -> bool {
72        let last = vec
73            .iter()
74            .copied()
75            .find(|x| *x != First(None))
76            .unwrap_or_default();
77        let right = vec
78            .iter()
79            .copied()
80            .fold(First::default(), |a, x| a.combine(x));
81
82        last == right
83    }
84
85    #[quickcheck]
86    fn associativity_property(x: First<u32>, y: First<u32>, z: First<u32>) -> bool {
87        x.combine(y.combine(z)) == x.combine(y).combine(z)
88    }
89}
partial_functional/monoid/first.rs

partial_functional/monoid/
first.rs
