[−][src]Trait approxeq::ApproxEq
Trait for equality comparisons that are approximately equal
This trait allows for approximate equality, for results that just have to be "good enough".
Herein a ~= b implies that a.aeq(b) and a !~= implies a.nae(b).
The approximate equality, however, must be (for all a, b and c)
symmetric, in that: a ~= b implies b ~= a.
How can I implement ApproxEq?
ApproxEq only requires the [aeq] method be implemented; [nae] is defined
in terms of it by default. Any implementation of [ane] must respect
the rule that [aeq] is a strict inverse of [ane]; that is, !(a ~= b) if
and only if a !~= b.
An example implementation for a domain in which two books are considered the same book if their ISBNs have the same parity, even if the formats differ:
use approxeq::ApproxEq; enum BookFormat { Paperback, Hardback, Ebook, } struct Book { isbn: i32, format: BookFormat, } impl ApproxEq for Book { fn aeq(&self, other: &Self) -> bool { self.isbn % 2 == other.isbn % 2 } } let b1 = Book { isbn: 3, format: BookFormat::Paperback }; let b2 = Book { isbn: 3, format: BookFormat::Ebook }; let b3 = Book { isbn: 10, format: BookFormat::Paperback }; assert!(b1.aeq(&b2)); assert!(b1.nae(&b3));
How can I compare two different types?
The type you can compare with is controlled by ApproxEq's type parameter.
For example, let's tweak our previous code a bit:
use approxeq::ApproxEq; #[derive(PartialEq)] enum BookFormat { Paperback, Hardback, Ebook, } struct Book { isbn: i32, format: BookFormat, } // Implement <Book> ~= <BookFormat> comparisons impl ApproxEq<BookFormat> for Book { fn aeq(&self, other: &BookFormat) -> bool { match self.format { BookFormat::Ebook => self.format == *other, _ => other != &BookFormat::Ebook, } } } // Implement <BookFormat> ~= <Book> comparisons impl ApproxEq<Book> for BookFormat { fn aeq(&self, other: &Book) -> bool { *self == BookFormat::Ebook && other.format == BookFormat::Ebook || *self != BookFormat::Ebook && other.format != BookFormat::Ebook } } let b1 = Book { isbn: 3, format: BookFormat::Paperback }; assert!(b1.aeq(&BookFormat::Paperback)); assert!(BookFormat::Ebook.nae(&b1));
By changing impl ApproxEq for Book to impl ApproxEq<BookFormat> for Book,
we allow BookFormats to be compared with Books.