X-Git-Url: https://git.ralfj.de/rust-101.git/blobdiff_plain/e2374eed1c3ae8d0063138ea011e86bbd42473ab..d9fc9a4d89d8e4950f43308318962d9179bd1177:/src/part07.rs diff --git a/src/part07.rs b/src/part07.rs index 92da555..85fe071 100644 --- a/src/part07.rs +++ b/src/part07.rs @@ -1,54 +1,149 @@ -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 borrows *of the same lifetime*, and then +// return a borrow of 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.
+// 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! 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`](http://doc.rust-lang.org/std/cmp/trait.PartialEq.html) +// and [`Eq`](http://doc.rust-lang.org/std/cmp/trait.Eq.html). `Eq` can be automatically derived as well. + +// Now we can compare `BigInt`s. Rust treats `PratialEq` special in that it is wired to the operator `==`: +// That operator can not 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](http://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`](http://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) | [next](main.html)