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// 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 //! } //! } //! } //! ``` /// 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 } } } #[cfg(test)] mod tests { 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 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 ); } }