predicates/
boxed.rs

1// Copyright (c) 2018 The predicates-rs Project Developers.
2//
3// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
4// http://www.apache.org/license/LICENSE-2.0> or the MIT license
5// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
6// option. This file may not be copied, modified, or distributed
7// except according to those terms.
8
9//! Predicate that can wrap other dynamically-called predicates in an
10//! easy-to-manage type.
11
12use std::fmt;
13
14use crate::reflection;
15use crate::Predicate;
16
17/// `Predicate` that wraps another `Predicate` as a trait object, allowing
18/// sized storage of predicate types.
19pub struct BoxPredicate<Item: ?Sized>(Box<dyn Predicate<Item> + Send + Sync>);
20
21impl<Item> BoxPredicate<Item>
22where
23    Item: ?Sized,
24{
25    /// Creates a new `BoxPredicate`, a wrapper around a dynamically-dispatched
26    /// `Predicate` type with useful trait impls.
27    pub fn new<P>(inner: P) -> BoxPredicate<Item>
28    where
29        P: Predicate<Item> + Send + Sync + 'static,
30    {
31        BoxPredicate(Box::new(inner))
32    }
33}
34
35impl<Item> fmt::Debug for BoxPredicate<Item>
36where
37    Item: ?Sized,
38{
39    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
40        f.debug_struct("BoxPredicate").finish()
41    }
42}
43
44impl<Item> reflection::PredicateReflection for BoxPredicate<Item>
45where
46    Item: ?Sized,
47{
48    fn parameters<'a>(&'a self) -> Box<dyn Iterator<Item = reflection::Parameter<'a>> + 'a> {
49        self.0.parameters()
50    }
51
52    fn children<'a>(&'a self) -> Box<dyn Iterator<Item = reflection::Child<'a>> + 'a> {
53        self.0.children()
54    }
55}
56
57impl<Item> fmt::Display for BoxPredicate<Item>
58where
59    Item: ?Sized,
60{
61    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
62        self.0.fmt(f)
63    }
64}
65
66impl<Item> Predicate<Item> for BoxPredicate<Item>
67where
68    Item: ?Sized,
69{
70    fn eval(&self, variable: &Item) -> bool {
71        self.0.eval(variable)
72    }
73
74    fn find_case<'a>(&'a self, expected: bool, variable: &Item) -> Option<reflection::Case<'a>> {
75        self.0.find_case(expected, variable)
76    }
77}
78
79/// `Predicate` extension for boxing a `Predicate`.
80pub trait PredicateBoxExt<Item: ?Sized>
81where
82    Self: Predicate<Item>,
83{
84    /// Returns a `BoxPredicate` wrapper around this `Predicate` type.
85    ///
86    /// Returns a `BoxPredicate` wrapper around this `Predicate` type. The
87    /// `BoxPredicate` type has a number of useful properties:
88    ///
89    ///   - It stores the inner predicate as a trait object, so the type of
90    ///     `BoxPredicate` will always be the same even if steps are added or
91    ///     removed from the predicate.
92    ///   - It is a common type, allowing it to be stored in vectors or other
93    ///     collection types.
94    ///   - It implements `Debug` and `Display`.
95    ///
96    /// # Examples
97    ///
98    /// ```
99    /// use predicates::prelude::*;
100    ///
101    /// let predicates = vec![
102    ///     predicate::always().boxed(),
103    ///     predicate::never().boxed(),
104    /// ];
105    /// assert_eq!(true, predicates[0].eval(&4));
106    /// assert_eq!(false, predicates[1].eval(&4));
107    /// ```
108    fn boxed(self) -> BoxPredicate<Item>
109    where
110        Self: Sized + Send + Sync + 'static,
111    {
112        BoxPredicate::new(self)
113    }
114}
115
116impl<P, Item: ?Sized> PredicateBoxExt<Item> for P where P: Predicate<Item> {}
117
118#[cfg(test)]
119mod test {
120    use crate::prelude::*;
121
122    #[test]
123    fn unsized_boxed() {
124        let p = predicate::always().boxed();
125        p.eval("4");
126    }
127
128    #[test]
129    fn boxed_find_case() {
130        let p1 = predicate::gt(5);
131        let p2 = p1.boxed();
132        match (p1.find_case(false, &4), p2.find_case(false, &4)) {
133            (Some(c1), Some(c2)) => {
134                assert_eq!(format!("{c1:?}"), format!("{c2:?}"));
135            }
136            _ => {
137                panic!();
138            }
139        }
140    }
141}
predicates/boxed.rs

predicates/
boxed.rs
