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;
pub fn vec_min<T: Minimum>(v: &Vec<T>) -> 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(++).<br/>
+//@ to check the pointer for validity, avoiding the safety issues of C(++). <br/>
//@ 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!()
// ## 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]
//@ 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);
// ## 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);
// 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
// **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<i32>` and `SomethingOrNothing<f32>`.
-//@ [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<i32>` and `SomethingOrNothing<f32>`.
+
+//@ [index](main.html) | [previous](part06.html) | [raw source](workspace/src/part07.rs) |
+//@ [next](part08.html)
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