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21 changes: 20 additions & 1 deletion library/core/src/option.rs
Original file line number Diff line number Diff line change
Expand Up @@ -1138,7 +1138,7 @@ impl<T> Option<T> {
/// Computes a default function result (if none), or
/// applies a different function to the contained value (if any).
///
/// # Examples
/// # Basic examples
///
/// ```
/// let k = 21;
Expand All @@ -1149,6 +1149,25 @@ impl<T> Option<T> {
/// let x: Option<&str> = None;
/// assert_eq!(x.map_or_else(|| 2 * k, |v| v.len()), 42);
/// ```
///
/// # Handling a Result-based fallback
///
/// A somewhat common occurrence when dealing with optional values
/// in combination with [`Result<T, E>`] is the case where one wants to invoke
/// a fallible fallback if the option is not present. This example
/// parses a command line argument (if present), or the contents of a file to
/// an integer. However, unlike accessing the command line argument, reading
/// the file is fallible, so it must be wrapped with `Ok`.
///
/// ```no_run
/// # fn main() -> Result<(), Box<dyn std::error::Error>> {
/// let v: u64 = std::env::args()
/// .nth(1)
/// .map_or_else(|| std::fs::read_to_string("/etc/someconfig.conf"), Ok)?
/// .parse()?;
/// # Ok(())
/// # }
/// ```
#[inline]
#[stable(feature = "rust1", since = "1.0.0")]
pub fn map_or_else<U, D, F>(self, default: D, f: F) -> U
Expand Down