karpal_core/
divisible.rs

1// Copyright (C) 2026 Industrial Algebra
2// SPDX-License-Identifier: Apache-2.0
3
4use crate::contravariant::PredicateF;
5use crate::divide::Divide;
6#[cfg(all(not(feature = "std"), feature = "alloc"))]
7use alloc::boxed::Box;
8
9/// Divisible: the contravariant analogue of Applicative.
10///
11/// Adds a `conquer` operation (the identity for `divide`), analogous to `pure`.
12///
13/// Laws:
14/// - Left identity: `divide(f, conquer(), fa) ≈ contramap(snd . f, fa)`
15/// - Right identity: `divide(f, fa, conquer()) ≈ contramap(fst . f, fa)`
16pub trait Divisible: Divide {
17    fn conquer<A: 'static>() -> Self::Of<A>;
18}
19
20impl Divisible for PredicateF {
21    fn conquer<A: 'static>() -> Box<dyn Fn(A) -> bool> {
22        Box::new(|_| true)
23    }
24}
25
26#[cfg(test)]
27mod tests {
28    use super::*;
29
30    #[test]
31    fn predicate_conquer() {
32        let p: Box<dyn Fn(i32) -> bool> = PredicateF::conquer();
33        assert!(p(42));
34        assert!(p(-1));
35        assert!(p(0));
36    }
37}
38
39#[cfg(test)]
40mod law_tests {
41    use super::*;
42    use proptest::prelude::*;
43
44    proptest! {
45        // Left identity: divide(|a| ((), a), conquer(), fa)(x) == fa(x)
46        #[test]
47        fn predicate_left_identity(x in any::<i32>()) {
48            let fa: Box<dyn Fn(i32) -> bool> = Box::new(|a| a > 0);
49            let expected = fa(x);
50
51            let fa2: Box<dyn Fn(i32) -> bool> = Box::new(|a| a > 0);
52            let result = PredicateF::divide(
53                |a: i32| ((), a),
54                PredicateF::conquer::<()>(),
55                fa2,
56            );
57            prop_assert_eq!(result(x), expected);
58        }
59
60        // Right identity: divide(|a| (a, ()), fa, conquer())(x) == fa(x)
61        #[test]
62        fn predicate_right_identity(x in any::<i32>()) {
63            let fa: Box<dyn Fn(i32) -> bool> = Box::new(|a| a > 0);
64            let expected = fa(x);
65
66            let fa2: Box<dyn Fn(i32) -> bool> = Box::new(|a| a > 0);
67            let result = PredicateF::divide(
68                |a: i32| (a, ()),
69                fa2,
70                PredicateF::conquer::<()>(),
71            );
72            prop_assert_eq!(result(x), expected);
73        }
74    }
75}
karpal_core/divisible.rs

karpal_core/
divisible.rs
