1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99

/// Fallback to an alternative, if the previous result outcome was a fail, and a predicate is true.
pub trait FallbackIf<R> {
    /// Fallback to an alternative when an outcome is considered a **fail** and **the predicate evaluates
    /// to true**, otherwise keep the current result.
    fn fallback_if<P, F>(self, predicate: P, alternative: F) -> R
    where
        P: Into<bool>,
        F: FnOnce() -> R;
}

impl<T, E> FallbackIf<Result<T, E>> for Result<T, E> {
    /// Fallback to an alternative when a result **produces an error** and **the predicate evaluates
    /// to true**, otherwise keep the current result.
    fn fallback_if<P, F>(self, predicate: P, alternative: F) -> Result<T, E>
    where
        P: Into<bool>,
        F: FnOnce() -> Result<T, E>,
    {
        if self.is_err() && predicate.into() {
            alternative()
        } else {
            self
        }
    }
}
impl<T> FallbackIf<Option<T>> for Option<T> {
    fn fallback_if<P, F>(self, predicate: P, alternative: F) -> Option<T>
    where
        P: Into<bool>,
        F: FnOnce() -> Option<T>,
    {
        if self.is_none() && predicate.into() {
            alternative()
        } else {
            self
        }
    }
}

#[cfg(test)]
mod tests {
    use super::*;

    #[yare::parameterized(
        ok =  { Ok(1), Ok(9), Ok(1) },                  // Does not enter the alternative, because input = Ok
        err_to_ok = { Err(()), Ok(9), Ok(9)  },         // Does enter the alternative, because input = Err and fallback pred. = true
        err_to_err = { Err(()), Err(()), Err(())  },    // Does enter the alternative, because input = Err and fallback pred. = true
    )]
    fn result_do_fallback(
        result: Result<u32, ()>,
        fallback_result: Result<u32, ()>,
        expected: Result<u32, ()>,
    ) {
        let outcome = result.fallback_if(true, || fallback_result);

        assert_eq!(outcome, expected)
    }

    #[yare::parameterized(
        ok =  { Ok(1), Ok(9), Ok(1) },                  // Does not enter the alternative, because input = Ok
        err_to_ok = { Err(()), Ok(9), Err(())  },       // Does not enter the alternative, because while input = Err, fallback pred. = false
        err_to_err = { Err(()), Err(()), Err(())  },    // Does not enter the alternative, because while input = Err, fallback pred. = false
    )]
    fn result_do_not_fallback(
        result: Result<u32, ()>,
        fallback_result: Result<u32, ()>,
        expected: Result<u32, ()>,
    ) {
        let outcome = result.fallback_if(false, || fallback_result);

        assert_eq!(outcome, expected)
    }

    #[yare::parameterized(
        ok =  { Some(1), Some(9), Some(1) },        // Does not enter the alternative, because input = Some
        none_to_some = { None, Some(9), Some(9)  }, // Does enter the alternative, because input = None and fallback pred. = true
        none_to_none = { None, None, None  },       // Does enter the alternative, because input = None and fallback pred. = true
    )]
    fn option_do_fallback(result: Option<u32>, fallback_value: Option<u32>, expected: Option<u32>) {
        let outcome = result.fallback_if(true, || fallback_value);

        assert_eq!(outcome, expected)
    }

    #[yare::parameterized(
        ok =  { Some(1), Some(9), Some(1) },        // Does not enter the alternative, because input = Some
        none_to_some = { None, Some(9), None  },    // Does not enter the alternative, because while input = None, fallback pred. = false
        none_to_none = { None, None, None  },       // Does not enter the alternative, because while input = None, fallback pred. = false
    )]
    fn option_do_not_fallback(
        result: Option<u32>,
        fallback_value: Option<u32>,
        expected: Option<u32>,
    ) {
        let outcome = result.fallback_if(false, || fallback_value);

        assert_eq!(outcome, expected)
    }
}