X-Git-Url: https://git.ralfj.de/rust-101.git/blobdiff_plain/4f61be32dd480f23a7fef05ee66c42ae27c980c6..refs/remotes/github/master:/src/part07.rs?ds=sidebyside diff --git a/src/part07.rs b/src/part07.rs index 6544995..b04d766 100644 --- a/src/part07.rs +++ b/src/part07.rs @@ -4,8 +4,8 @@ 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 +//@ 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 { fn min<'a>(&'a self, other: &'a Self) -> &'a Self; @@ -17,25 +17,26 @@ pub trait Minimum { pub fn vec_min(v: &Vec) -> Option<&T> { let mut min: Option<&T> = None; for e in v { - min = Some(match min { /*@*/ - None => e, /*@*/ - Some(n) => n.min(e) /*@*/ - }); /*@*/ + min = Some(match min { + None => e, + Some(n) => n.min(e) + }); } min } //@ 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(++).
+//@ 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! +//@ `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!() @@ -44,13 +45,15 @@ impl Minimum for BigInt { // ## 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. +//@ 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] @@ -62,28 +65,34 @@ impl PartialEq for BigInt { //@ 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). +//@ 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`. +//@ 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. +//@ 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); @@ -92,8 +101,9 @@ fn compare_big_ints() { // ## 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. +//@ 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); @@ -106,15 +116,16 @@ fn test_min() { // 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. +//@ 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 @@ -142,9 +153,11 @@ fn test_vec_min() { // **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) +// **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](workspace/src/part07.rs) | +//@ [next](part08.html)