fix the main file(s)

[rust-101.git] / src / part07.rs

diff --git a/src/part07.rs b/src/part07.rs
index a996a22d5cf0e3217dfbc942db0d11dab41cc045..85fe071a6317ff34f8328c1028eaa3617186af44 100644 (file)
--- a/src/part07.rs
+++ b/src/part07.rs
@@ -13,7 +13,7 @@ pub trait Minimum {
 
 // 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
 
 // 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.
+// lifetimes actually work out. Observe that we don't have to make any copies!

 pub fn vec_min<T: Minimum>(v: &Vec<T>) -> Option<&T> {
     let mut min: Option<&T> = None;
     for e in v {
 pub fn vec_min<T: Minimum>(v: &Vec<T>) -> Option<&T> {
     let mut min: Option<&T> = None;
     for e in v {


@@ -27,16 +27,15 @@ pub fn vec_min<T: Minimum>(v: &Vec<T>) -> Option<&T> {
 // 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
 // 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(++). 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 the `NULL`-check that Java does all the time.<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.
 
 // 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.
 

-// For our `vec_min` to be usable with `BigInt`, we need to provide an implementation of
-// `Minimum`. You should be able to pretty much copy the code you wrote for exercise 06.1.
+// **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!()
 impl Minimum for BigInt {
     fn min<'a>(&'a self, other: &'a Self) -> &'a Self {
         unimplemented!()


@@ -45,12 +44,13 @@ impl Minimum for BigInt {
 
 // ## Operator Overloading
 // How can we know that our `min` function actually does what we want it to do? One possibility
 
 // ## 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 build-in support for both unit tests and integration
+// 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`.
 
 // 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.
+// 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]
 // The `inline` attribute tells Rust that we will typically want this function to be inlined.
 impl PartialEq for BigInt {
     #[inline]


@@ -64,7 +64,8 @@ impl PartialEq for BigInt {
 // 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.
 
 // 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 using `==`! Speaking in C++ terms, we just overloaded the `==` operator
+// 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
 // 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


@@ -75,6 +76,13 @@ impl PartialEq for BigInt {
 // 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.
 // 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);
 fn compare_big_ints() {
     let b1 = BigInt::new(13);
     let b2 = BigInt::new(37);


@@ -135,6 +143,7 @@ fn test_vec_min() {
 // 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,
 // 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.
+// 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)
 
 // [index](main.html) | [previous](part06.html) | [next](main.html)