if_empty/

lib.rs

// This Source Code Form is subject to the terms of the Mozilla Public
// License, v. 2.0. If a copy of the MPL was not distributed with this
// file, You can obtain one at http://mozilla.org/MPL/2.0/.

//! For use with defensive programming where context specific defaults are needed.
//!
//! While using an [`Option`] is preferrably in most circumstances there are situations where a function call doesn't return an
//! [`Option`] and the [`Default`] of a type isn't helpful either.
//!
//! [`Option`]: https://doc.rust-lang.org/std/option/enum.Option.html
//! [`Default`]: https://doc.rust-lang.org/std/default/trait.Default.html
//! [`str`]: https://doc.rust-lang.org/std/primitive.str.html
//! [`String`]: https://doc.rust-lang.org/std/string/struct.String.html
//! [`String::is_empty()`]: https://doc.rust-lang.org/nightly/alloc/string/struct.String.html#method.is_empty
//! [`str::is_empty()`]: https://doc.rust-lang.org/std/primitive.str.html#method.is_empty
//!
//! # Examples
//!
//! ```
//! # struct Bar {}
//! # impl Bar {
//! #     fn is_empty(&self) -> bool { true }
//! # }
//! # fn bar() -> Bar {
//! #     Bar {}
//! # }
//! //  Converting
//! let foo = {
//!    let b = bar();
//!    if b.is_empty() {
//!        Bar {
//!            // set the default values for your context here
//!        }
//!    } else {
//!        b
//!    }
//! };
//! // into
//! use if_empty::IfEmpty;
//! impl IfEmpty for Bar {
//!      fn if_empty(self, value: Self) -> Self {
//!          // implement
//! #         if self.is_empty() {
//! #             value
//! #         } else {
//! #             self
//! #         }
//!      }
//! }
//!
//! let foo = bar().if_empty(Bar { /* ... */ });
//! ```
//!
//! # Implementation
//!
//! In this example we're using the obvious `is_empty()` function for `Foo` but we could also
//! do more elaborate checks.
//!
//! ```
//! use if_empty::IfEmpty;
//!
//! struct Foo {
//!    val: bool,
//! }
//!
//! impl Foo {
//!     fn is_empty(&self) -> bool { !self.val }
//! }
//!
//! impl IfEmpty for Foo {
//!    fn if_empty(self, value: Foo) -> Foo {
//!        if self.is_empty() {
//!            value
//!        } else {
//!            self
//!        }
//!    }
//! }
//! ```

pub use if_empty_derive::IfEmpty;

/// For checking IfEmpty on value semantics
pub trait IfEmpty {
    /// Returns `val` if the `self` is empty
    fn if_empty(self, val: Self) -> Self;
}

/// For checking IfEmpty on borrowed objects
pub trait IfEmptyBorrowed {
    /// Return `val` if `self` is empty
    fn if_empty<'a>(&'a self, val: &'a Self) -> &'a Self;
}

/// Implementation of `IfEmptyBorrowed` for [`str`]
impl IfEmptyBorrowed for str {
    /// Returns `input` if [`str::is_empty()`] returns true.
    /// Otherwise `self` is returned.
    fn if_empty<'a>(&'a self, input: &'a Self) -> &'a Self {
        if self.is_empty() {
            input
        } else {
            self
        }
    }
}

/// Implementation of `IfEmpty` for [`String`]
impl IfEmpty for String {
    /// Returns `input` if [`String::is_empty()`] returns true.
    /// Otherwise `self` is returned.
    fn if_empty(self, input: Self) -> Self {
        if self.is_empty() {
            input
        } else {
            self
        }
    }
}

/// [`OsStr`]: https://doc.rust-lang.org/std/ffi/struct.OsStr.html
/// Implementation of `IfEmptyBorrowed` for [`OsStr`]
impl IfEmptyBorrowed for std::ffi::OsStr {
    /// [`OsStr::is_empty()`]: https://doc.rust-lang.org/std/ffi/struct.OsStr.html#method.is_empty
    /// Returns `input` if [`OsStr::is_empty()`] returns true.
    /// Otherwise `self` is returned.
    fn if_empty<'a>(&'a self, input: &'a Self) -> &'a Self {
        if self.is_empty() {
            input
        } else {
            self
        }
    }
}

/// [`OsString`]: https://doc.rust-lang.org/std/ffi/struct.OsString.html
/// Implementation of `IfEmpty` for [`OsString`]
impl IfEmpty for std::ffi::OsString {
    /// [`OsString::is_empty()`]: https://doc.rust-lang.org/std/ffi/struct.OsString.html#method.is_empty
    /// Returns `input` if [`OsString::is_empty()`] returns true.
    /// Otherwise `self` is returned.
    fn if_empty(self, input: Self) -> Self {
        if self.is_empty() {
            input
        } else {
            self
        }
    }
}

#[cfg(test)]
mod tests {
    use std::ffi::{OsStr, OsString};

    use crate::{IfEmpty, IfEmptyBorrowed};

    #[test]
    fn string() {
        let string = String::default();
        assert!(string.is_empty());
        let replacement = "text".to_string();
        let replaced = string.if_empty(replacement.clone());
        assert!(!replaced.is_empty());
        assert_eq!(replacement, replaced);

        let string = "not empty".to_string();
        assert!(!string.is_empty());
        assert_eq!("not empty", string.if_empty("should not be returned".to_string()));
    }
    #[test]
    fn str() {
        let string: &str = "";
        assert!(string.is_empty());
        let replacement = "text";
        let replaced = string.if_empty(replacement);
        assert!(!replaced.is_empty());
        assert_eq!(replacement, replaced);

        let string: &str = "not empty";
        assert!(!string.is_empty());
        assert_eq!("not empty", string.if_empty("should not be returned"));
    }
    #[test]
    fn os_string() {
        let string = OsString::default();
        assert!(string.is_empty());
        let replacement = OsString::from("text");
        let replaced = string.if_empty(replacement.clone());
        assert!(!replaced.is_empty());
        assert_eq!(replacement, replaced);

        let string = OsString::from("not empty");
        assert!(!string.is_empty());
        assert_eq!(
            OsString::from("not empty"),
            string.if_empty(OsString::from("should not be returned"))
        );
    }
    #[test]
    fn os_str() {
        let string = OsStr::new("");
        assert!(string.is_empty());
        let replacement = OsStr::new("text");
        let replaced = string.if_empty(replacement);
        assert!(!replaced.is_empty());
        assert_eq!(replacement, replaced);

        let string = OsStr::new("not empty");
        assert!(!string.is_empty());
    }
    #[test]
    fn custom() {
        struct Fake {
            value: bool,
        }

        impl IfEmpty for Fake {
            fn if_empty(self, value: Self) -> Self {
                if self.value {
                    self
                } else {
                    value
                }
            }
        }

        let f = Fake {
            value: false,
        };
        assert!(
            f.if_empty(Fake {
                value: true
            })
            .value
        );

        let f = Fake {
            value: true,
        };
        assert!(
            f.if_empty(Fake {
                value: false
            })
            .value
        );
    }

    #[test]
    fn derive_macro() {
        #[derive(IfEmpty)]
        struct Example {
            value: String,
        }

        impl Example {
            fn is_empty(&self) -> bool {
                self.value.is_empty()
            }
        }

        let e = Example {
            value: String::new(),
        };
        assert_eq!(
            e.if_empty(Example {
                value: "not empty".to_string(),
            })
            .value,
            "not empty"
        );

        let e = Example {
            value: "a string".to_string(),
        };
        assert_eq!(
            e.if_empty(Example {
                value: "not empty".to_string(),
            })
            .value,
            "a string"
        );
    }
}