X-Git-Url: https://git.ralfj.de/rust-101.git/blobdiff_plain/e2374eed1c3ae8d0063138ea011e86bbd42473ab..706bf6cb37885ca97a49f772de00b535cf3dbf9f:/src/part07.rs?ds=sidebyside diff --git a/src/part07.rs b/src/part07.rs index 92da555..2d88390 100644 --- a/src/part07.rs +++ b/src/part07.rs @@ -1,54 +1,150 @@ -use part05::BigInt; +// Rust-101, Part 07: Operator Overloading, Tests, Formatting +// ========================================================== +pub use part05::BigInt; + +// With our new knowledge of lifetimes, we are now able to write down the desired type of `min`: +//@ We want the function to take two references *with the same lifetime*, and then +//@ return a reference with that lifetime. If the two input lifetimes would be different, we +//@ would not know which lifetime to use for the result. pub trait Minimum { - /// Return the smaller of the two fn min<'a>(&'a self, other: &'a Self) -> &'a Self; } -/// Return a pointer to the minimal value of `v`. +//@ Now we can implement a generic function `vec_min` that works on above trait. +//@ The code is pretty much straight-forward, and Rust checks that all the +//@ lifetimes actually work out. Observe that we don't have to make any copies! pub fn vec_min(v: &Vec) -> Option<&T> { - let mut min = None; + let mut min: Option<&T> = None; for e in v { min = Some(match min { None => e, - Some(n) => e.min(n) + Some(n) => n.min(e) }); } min } -// Notice that `Option<&T>` is technically (leaving the borrowing story aside) a pointer to a `T`, -// that could optionally be invalid. In other words, it's just like a pointer in C(++) or Java -// that can be `NULL`! However, thanks to `Option` being an `enum`, we cannot forget to check -// the pointer for validity, avoiding the safety issues of C(++). At the same time, when we -// have a borrow like `v` above that's not an `Option`, we *know* that is has to be a valid -// pointer, so we don't even need to do a `NULL`-check.
-// Also, if you are worried about wasting space, notice that Rust knows that `&T` can never be -// `NULL`, and hence optimizes `Option<&T>` to be no larger than `&T`. The `None` case is represented -// as `NULL`. This is another great example of a zero-cost abstraction: `Option<&T>` is exactly like -// a pointer in C(++), if you look at what happens during execution - but it's much safer to use. +//@ Notice that the return type `Option<&T>` is technically (leaving the borrowing story aside) a +//@ pointer to a `T`, that could optionally be invalid. In other words, it's just like a pointer in +//@ C(++) or Java that can be `NULL`! However, thanks to `Option` being an `enum`, we cannot forget +//@ to check the pointer for validity, avoiding the safety issues of C(++).
+//@ Also, if you are worried about wasting space, notice that Rust knows that `&T` can never be +//@ `NULL`, and hence optimizes `Option<&T>` to be no larger than `&T`. The `None` case is represented +//@ as `NULL`. This is another great example of a zero-cost abstraction: `Option<&T>` is exactly like +//@ a pointer in C(++), if you look at what happens during execution - but it's much safer to use. +// **Exercise 07.1**: For our `vec_min` to be usable with `BigInt`, you will have to provide an implementation of +// `Minimum`. You should be able to pretty much copy the code you wrote for exercise 06.1. You should *not* +// make any copies of `BigInt`! impl Minimum for BigInt { fn min<'a>(&'a self, other: &'a Self) -> &'a Self { + unimplemented!() + } +} + +// ## Operator Overloading +//@ How can we know that our `min` function actually does what we want it to do? One possibility +//@ here is to do *testing*. Rust comes with nice built-in support for both unit tests and integration +//@ tests. However, before we go there, we need to have a way of checking whether the results of function calls are +//@ correct. In other words, we need to define how to test equality of `BigInt`. Being able to +//@ test equality is a property of a type, that - you guessed it - Rust expresses as a trait: `PartialEq`. + +//@ Doing this for `BigInt` is fairly easy, thanks to our requirement that there be no trailing zeros. We simply +//@ re-use the equality test on vectors, which compares all the elements individually. +//@ The `inline` attribute tells Rust that we will typically want this function to be inlined. +impl PartialEq for BigInt { + #[inline] + fn eq(&self, other: &BigInt) -> bool { debug_assert!(self.test_invariant() && other.test_invariant()); - if self.data.len() < other.data.len() { - self - } else if self.data.len() > other.data.len() { - other - } else { - // compare back-to-front, i.e., most significant digit first - let mut idx = self.data.len()-1; - while idx > 0 { - if self.data[idx] < other.data[idx] { - return self; - } else if self.data[idx] > other.data[idx] { - return other; - } - else { - idx = idx-1; - } - } - // the two are equal - return self; - } + self.data == other.data /*@*/ } } + +//@ Since implementing `PartialEq` is a fairly mechanical business, you can let Rust automate this +//@ by adding the attribute `derive(PartialEq)` to the type definition. In case you wonder about +//@ the "partial", I suggest you check out the documentation of [`PartialEq`](https://doc.rust-lang.org/std/cmp/trait.PartialEq.html) +//@ and [`Eq`](https://doc.rust-lang.org/std/cmp/trait.Eq.html). `Eq` can be automatically derived as well. + +// Now we can compare `BigInt`s. Rust treats `PartialEq` special in that it is wired to the operator `==`: +//@ That operator can now be used on our numbers! Speaking in C++ terms, we just overloaded the `==` operator +//@ for `BigInt`. Rust does not have function overloading (i.e., it will not dispatch to different +//@ functions depending on the type of the argument). Instead, one typically finds (or defines) a +//@ trait that catches the core characteristic common to all the overloads, and writes a single +//@ function that's generic in the trait. For example, instead of overloading a function for all +//@ the ways a string can be represented, one writes a generic functions over [ToString](https://doc.rust-lang.org/std/string/trait.ToString.html). +//@ Usually, there is a trait like this that fits the purpose - and if there is, this has the great +//@ advantage that any type *you* write, that can convert to a string, just has to implement +//@ that trait to be immediately usable with all the functions out there that generalize over `ToString`. +//@ Compare that to C++ or Java, where the only chance to add a new overloading variant is to +//@ edit the class of the receiver. +//@ +//@ Why can we also use `!=`, even though we just overloaded `==`? The answer lies in what's called a *default implementation*. +//@ If you check out the documentation of `PartialEq` I linked above, you will see that the trait actually provides +//@ two methods: `eq` to test equality, and `ne` to test inequality. As you may have guessed, `!=` is wired to `ne`. +//@ The trait *definition* also provides a default implementation of `ne` to be the negation of `eq`. Hence you can just +//@ provide `eq`, and `!=` will work fine. Or, if you have a more efficient way of deciding inequality, you can provide +//@ `ne` for your type yourself. +fn compare_big_ints() { + let b1 = BigInt::new(13); + let b2 = BigInt::new(37); + println!("b1 == b1: {} ; b1 == b2: {}; b1 != b2: {}", b1 == b1, b1 == b2, b1 != b2); +} + +// ## Testing +// With our equality test written, we are now ready to write our first testcase. +//@ It doesn't get much simpler: You just write a function (with no arguments or return value), and give it +// the `test` attribute. `assert!` is like `debug_assert!`, but does not get compiled away in a release build. +#[test] +fn test_min() { + let b1 = BigInt::new(1); + let b2 = BigInt::new(42); + let b3 = BigInt::from_vec(vec![0, 1]); + + assert!(*b1.min(&b2) == b1); /*@*/ + assert!(*b3.min(&b2) == b2); /*@*/ +} +// Now run `cargo test` to execute the test. If you implemented `min` correctly, it should all work! + +// ## Formatting +//@ There is also a macro `assert_eq!` that's specialized to test for equality, and that prints the two +//@ values (left and right) if they differ. To be able to do that, the macro needs to know how to format +//@ the value for printing. This means that we - guess what? - have to implement an appropriate trait. +//@ Rust knows about two ways of formatting a value: `Display` is for pretty-printing something in a way +//@ that users can understand, while `Debug` is meant to show the internal state of data and targeted at +//@ the programmer. The latter is what we want for `assert_eq!`, so let's get started. + +// All formating is handled by [`std::fmt`](https://doc.rust-lang.org/std/fmt/index.html). I won't explain +// all the details, and refer you to the documentation instead. +use std::fmt; + +//@ In the case of `BigInt`, we'd like to just output our internal `data` array, so we +//@ simply call the formating function of `Vec`. +impl fmt::Debug for BigInt { + fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result { + self.data.fmt(f) + } +} +//@ `Debug` implementations can be automatically generated using the `derive(Debug)` attribute. + +// Now we are ready to use `assert_eq!` to test `vec_min`. +/*#[test]*/ +fn test_vec_min() { + let b1 = BigInt::new(1); + let b2 = BigInt::new(42); + let b3 = BigInt::from_vec(vec![0, 1]); + + let v1 = vec![b2.clone(), b1.clone(), b3.clone()]; + let v2 = vec![b2.clone(), b3.clone()]; + assert_eq!(vec_min(&v1), Some(&b1)); /*@*/ + assert_eq!(vec_min(&v2), Some(&b2)); /*@*/ +} + +// **Exercise 07.1**: Add some more testcases. In particular, make sure you test the behavior of +// `vec_min` on an empty vector. Also add tests for `BigInt::from_vec` (in particular, removing +// trailing zeros). Finally, break one of your functions in a subtle way and watch the test fail. +// +// **Exercise 07.2**: Go back to your good ol' `SomethingOrNothing`, and implement `Display` for it. (This will, +// of course, need a `Display` bound on `T`.) Then you should be able to use them with `println!` just like you do +// with numbers, and get rid of the inherent functions to print `SomethingOrNothing` and `SomethingOrNothing`. + +//@ [index](main.html) | [previous](part06.html) | [raw source](https://www.ralfj.de/git/rust-101.git/blob_plain/HEAD:/workspace/src/part07.rs) | [next](part08.html)