Merge #593

593: EitherOrBoth: Add or and or_else methods to simplify getting default values r=phimuemue a=carl-anders

## Introducing `or` and `or_else` methods to `EitherOrBoth`
This feature reduces the amount of code required to get specific default values when using EitherOrBoth.
An example of the current way of using `zip_longest` with custom default values:
```rust
let a = (0..=4).into_iter();
let b = (0..=2).into_iter();
let c = a.zip_longest(b).map(|e| match e {
    EitherOrBoth::Both(l, r) => (l, r),
    EitherOrBoth::Left(l) => (l, 2),
    EitherOrBoth::Right(r) => (4, r),
});
// c will now contain an iterator with (0,0), (1,1), (2,2), (3,2), (4,2).
```
An example with the proposed `or` method:
```rust
let a = (0..=4).into_iter();
let b = (0..=2).into_iter();
let c = a.zip_longest(b).map(|e| e.or(4, 2));
// c will now contain an iterator with (0,0), (1,1), (2,2), (3,2), (4,2).
```
I have also included the `or_else` method which does the same but with closures.

## Contribute questions
> Include tests for your new feature, preferably a QuickCheck test

There are no tests for the other `EitherOrBoth` methods, so I was unsure how to place them. However, I have added **DocTest**'s to both methods. These examples allows for easier to understand documentation, and also tests the validity of the methods.

> For new features, please first consider filing a PR to rust-lang/rust

The EitherOrBoth struct does not exist in rust std library.

## Concerns

The naming is slightly inconsistent when compared to rust's `std::Option`, considering the naming from there the added methods should be named `unwrap_or` and `unwrap_or_else`.
However, this would then become inconsistent with the existing method `EitherOrBoth::or_default`. Which is why I went with the chosen names.
I can change the method names if needed, but then we should consider changing `or_default` too.

## P.S.
The `CHANGELOG.md` file has the text `Add EitherOrBoth::or_default (#583)`. This number is wrong, it should be #538.


Co-authored-by: Carl Andersson <carl@carlandersson.net>

Author: bors[bot]

src/either_or_both.rs

@@ -164,6 +164,33 @@ impl<A, B> EitherOrBoth<A, B> {
         }
     }
 
+    /// Returns a tuple consisting of the `l` and `r` in `Both(l, r)`, if present.
+    /// Otherwise, returns the wrapped value for the present element, and the supplied
+    /// value for the other. The first (`l`) argument is used for a missing `Left`
+    /// value. The second (`r`) argument is used for a missing `Right` value.
+    ///
+    /// Arguments passed to `or` are eagerly evaluated; if you are passing
+    /// the result of a function call, it is recommended to use [`or_else`],
+    /// which is lazily evaluated.
+    ///
+    /// [`or_else`]: EitherOrBoth::or_else
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// # use itertools::EitherOrBoth;
+    /// assert_eq!(EitherOrBoth::Both("tree", 1).or("stone", 5), ("tree", 1));
+    /// assert_eq!(EitherOrBoth::Left("tree").or("stone", 5), ("tree", 5));
+    /// assert_eq!(EitherOrBoth::Right(1).or("stone", 5), ("stone", 1));
+    /// ```
+    pub fn or(self, l: A, r: B) -> (A, B) {
+        match self {
+            Left(inner_l) => (inner_l, r),
+            Right(inner_r) => (l, inner_r),
+            Both(inner_l, inner_r) => (inner_l, inner_r),
+        }
+    }
+
     /// Returns a tuple consisting of the `l` and `r` in `Both(l, r)`, if present.
     /// Otherwise, returns the wrapped value for the present element, and the [`default`](Default::default)
     /// for the other.
@@ -178,6 +205,28 @@ impl<A, B> EitherOrBoth<A, B> {
             EitherOrBoth::Both(l, r) => (l, r),
         }
     }
+
+    /// Returns a tuple consisting of the `l` and `r` in `Both(l, r)`, if present.
+    /// Otherwise, returns the wrapped value for the present element, and computes the
+    /// missing value with the supplied closure. The first argument (`l`) is used for a
+    /// missing `Left` value. The second argument (`r`) is used for a missing `Right` value.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// # use itertools::EitherOrBoth;
+    /// let k = 10;
+    /// assert_eq!(EitherOrBoth::Both("tree", 1).or_else(|| "stone", || 2 * k), ("tree", 1));
+    /// assert_eq!(EitherOrBoth::Left("tree").or_else(|| "stone", || 2 * k), ("tree", 20));
+    /// assert_eq!(EitherOrBoth::Right(1).or_else(|| "stone", || 2 * k), ("stone", 1));
+    /// ```
+    pub fn or_else<L: FnOnce() -> A, R: FnOnce() -> B>(self, l: L, r: R) -> (A, B) {
+        match self {
+            Left(inner_l) => (inner_l, r()),
+            Right(inner_r) => (l(), inner_r),
+            Both(inner_l, inner_r) => (inner_l, inner_r),
+        }
+    }
 }
 
 impl<T> EitherOrBoth<T, T> {