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//! Extension traits for `std::Result`.

/// Extension trait with useful methods for [`std::result::Result`].
///
/// [`std::result::Result`]: https://doc.rust-lang.org/std/result/enum.Result.html
pub trait ResultExt<T, E> {
/// Combines `self` and another `Result`.
///
/// If `self` is `Ok(s)` and `other` is `Ok(o)`, this method returns `Ok((s, o))`.
/// Otherwise, if the `self` is `Ok(s)` and `other` is `Err(e)`, this method returns `Err(e)`.
/// Otherwise, `self` is `Err(e)` and this method returns `Err(e)` (`other` is not taken into
/// account, as in short circuit calculations).
///
/// # Examples
///
/// ```
/// use stdext::prelude::*;
///
/// let x = Ok(1);
/// let y = Ok("hi");
/// let z: Result<i32, &str> = Err("error");
/// let z2: Result<i32, &str> = Err("other_error");
///
/// assert_eq!(x.combine(y), Ok((1, "hi")));
/// assert_eq!(x.combine(z), Err("error"));
/// assert_eq!(z.combine(z2), Err("error"));
/// ```
fn combine<U>(self, other: Result<U, E>) -> Result<(T, U), E>;

/// Combines `self` and another `Result` with function `f`.
///
/// If `self` is `Ok(s)` and `other` is `Ok(o)`, this method returns `Ok(f(s, o))`.
/// Otherwise, if the `self` is `Ok(s)` and `other` is `Err(e)`, this method returns `Err(e)`.
/// Otherwise, `self` is `Err(e)` and this method returns `Err(e)` (`other` is not taken into
/// account, as in short circuit calculations).
///
/// # Examples
///
/// ```
/// use stdext::prelude::*;
///
/// let x = Ok(1);
/// let y = Ok(2);
/// let z: Result<i32, &str> = Err("error");
/// let z2: Result<i32, &str> = Err("other_error");
///
/// assert_eq!(x.combine_with(y, |l, r| l + r), Ok(3));
/// assert_eq!(x.combine_with(z, |l, r| l + r), Err("error"));
/// assert_eq!(z.combine_with(z2, |l, r| l + r), Err("error"));
/// ```
///
/// [`zip_with`]: https://doc.rust-lang.org/std/Result/enum.Result.html#method.zip_with
fn combine_with<U, F, R>(self, other: Result<U, E>, f: F) -> Result<R, E>
where
F: FnOnce(T, U) -> R;
}

impl<T, E> ResultExt<T, E> for Result<T, E> {
fn combine<U>(self, other: Result<U, E>) -> Result<(T, U), E> {
match (self, other) {
(Ok(left), Ok(right)) => Ok((left, right)),
(Ok(_), Err(err)) => Err(err),
(Err(err), _) => Err(err),
}
}

fn combine_with<U, F, R>(self, other: Result<U, E>, f: F) -> Result<R, E>
where
F: FnOnce(T, U) -> R,
{
self.combine(other).map(|(l, r)| f(l, r))
}
}

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

#[test]
fn combine() {
// Test vector of (left, right, expected) values.
let test_vector = vec![
(Ok(1), Ok(2), Ok((1, 2))),
(Ok(1), Err("right"), Err("right")),
(Err("left"), Ok(2), Err("left")),
(Err("left"), Err("right"), Err("left")),
];

for (left, right, expected) in test_vector {
assert_eq!(left.combine(right), expected);
}
}

#[test]
fn combine_with() {
fn f(l: i32, r: i32) -> i32 {
l + r
};

// Test vector of (left, right, expected) values.
let test_vector = vec![
(Ok(1), Ok(2), Ok(3)),
(Ok(1), Err("right"), Err("right")),
(Err("left"), Ok(2), Err("left")),
(Err("left"), Err("right"), Err("left")),
];

for (left, right, expected) in test_vector {
assert_eq!(left.combine_with(right, f), expected);
}
}
}