add boolean operator example

Author: tlyu

core/src/option.rs

@@ -233,6 +233,36 @@
 //! [`or_else`]: Option::or_else
 //! [`xor`]: Option::xor
 //!
+//! This is an example of using methods like [`and_then`] and [`or`] in a
+//! pipeline of method calls. Early stages of the pipeline pass failure
+//! values ([`None`]) through unchanged, and continue processing on
+//! success values ([`Some`]). Toward the end, [`or`] substitutes an error
+//! message if it receives [`None`].
+//!
+//! ```
+//! # use std::collections::BTreeMap;
+//! let mut bt = BTreeMap::new();
+//! bt.insert(20u8, "foo");
+//! bt.insert(42u8, "bar");
+//! let res = vec![0u8, 1, 11, 200, 22]
+//!     .into_iter()
+//!     .map(|x| {
+//!         // `checked_sub()` returns `None` on error
+//!         x.checked_sub(1)
+//!             // same with `checked_mul()`
+//!             .and_then(|x| x.checked_mul(2))
+//!             // `BTreeMap::get` returns `None` on error
+//!             .and_then(|x| bt.get(&x))
+//!             // Substitute an error message if we have `None` so far
+//!             .or(Some(&"error!"))
+//!             .copied()
+//!             // Won't panic because we unconditionally used `Some` above
+//!             .unwrap()
+//!     })
+//!     .collect::<Vec<_>>();
+//! assert_eq!(res, ["error!", "error!", "foo", "error!", "bar"]);
+//! ```
+//!
 //! ## Iterating over `Option`
 //!
 //! An [`Option`] can be iterated over. This can be helpful if you need an