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#![deny(missing_docs)]

//! This crate adds a `.filter()` method to `Option<T>`, for older
//! versions of Rust that don't provide it.
//!
//! **Note: [`Option::filter`][std] was added to the standard library in
//! [Rust 1.27][rust]. Unless you need to support older versions of
//! Rust, you do not need to use this crate.**
//!
//! [std]: https://doc.rust-lang.org/std/option/enum.Option.html#method.filter
//! [rust]: https://github.com/rust-lang/rust/blob/master/RELEASES.md#version-1270-2018-06-21
//!
//! To use it, add `option-filter` to your `Cargo.toml`:
//!
//! ```toml
//! [dependencies]
//! option-filter = "1.0"
//! ```
//!
//! Then import the extension trait:
//!
//! ```rust,ignore
//! extern crate option_filter;
//! use option_filter::OptionFilterExt;
//! ```
//!
//! Now you can filter your `Option`s!
//!
//! ```rust
//! # use option_filter::OptionFilterExt;
//! let answer = Some(42);
//! assert_eq!(answer.filter(|x| *x == 42), Some(42));
//! assert_eq!(answer.filter(|x| *x == 43), None);
//! ```

/// Extension trait for adding a `.filter()` method to `Option<T>`.
///
/// This trait is intended for extending `Option<T>` only, and should
/// not be implemented for other types.
pub trait OptionFilterExt {
    /// The inner type of the `Option`.
    ///
    /// This is given an unwieldy name so that it's unlikely to conflict
    /// with other associated types.
    type OptionFilterInner;

    /// Filters the element of an `Option`.
    fn filter<F: FnOnce(&Self::OptionFilterInner) -> bool>(self, F) -> Self;
}

impl<T> OptionFilterExt for Option<T> {
    type OptionFilterInner = T;
    fn filter<F: FnOnce(&T) -> bool>(self, callback: F) -> Option<T> {
        match self {
            None => None,
            Some(x) => if callback(&x) { Some(x) } else { None }
        }
    }
}