document intended usage of 'workspace'

[rust-101.git] / src / part06.rs

diff --git a/src/part06.rs b/src/part06.rs
index 26fa1244d3c147585637b0ef9c2699ac85344245..e159ca5582ed9b2326c0884b2805509fc1fb7749 100644 (file)
--- a/src/part06.rs
+++ b/src/part06.rs
@@ -1,100 +1,100 @@
-// Rust-101, Part 06: Copy
-// =======================
+// Rust-101, Part 06: Copy, Lifetimes
+// ==================================

 
 

+// We continue to work on our `BigInt`, so we start by importing what we already established.

 use part05::BigInt;
 
 // With `BigInt` being about numbers, we should be able to write a version of `vec_min`
 use part05::BigInt;
 
 // With `BigInt` being about numbers, we should be able to write a version of `vec_min`

-// that computes the minimum of a list of `BigInt`. We start by writing `min` for
-// `BigInt`. Now our assumption of having no trailing zeros comes in handy!
+// that computes the minimum of a list of `BigInt`. First, we have to write `min` for `BigInt`.

 impl BigInt {
 impl BigInt {

-    fn min(self, other: Self) -> Self {
-        // Just to be sure, we first check that both operands actually satisfy our invariant.
-        // `debug_assert!` is a macro that checks that its argument (must be of type `bool`)
-        // is `true`, and panics otherwise. It gets removed in release builds, which you do with
-        // `cargo build --release`.
+    fn min_try1(self, other: Self) -> Self {
+        // Just to be sure, we first check that both operands actually satisfy our invariant. `debug_assert!` is a
+        // macro that checks that its argument (must be of type `bool`) is `true`, and panics otherwise. It gets
+        // removed in release builds, which you do with `cargo build --release`.

         debug_assert!(self.test_invariant() && other.test_invariant());
         debug_assert!(self.test_invariant() && other.test_invariant());

+        // Now our assumption of having no trailing zeros comes in handy:

         // If the lengths of the two numbers differ, we already know which is larger.
         if self.data.len() < other.data.len() {
             self
         } else if self.data.len() > other.data.len() {
             other
         } else {
         // If the lengths of the two numbers differ, we already know which is larger.
         if self.data.len() < other.data.len() {
             self
         } else if self.data.len() > other.data.len() {
             other
         } else {

-            // **Exercise 05.1**: Fill in this code.
-            panic!("Not yet implemented.");
+            // **Exercise 06.1**: Fill in this code.
+            unimplemented!()

         }
     }
 }
 
         }
     }
 }
 

-// Now we can write `vec_min`. In order to make it type-check, we have to write it as follows.
+// Now we can write `vec_min`. In order to make it type-check, we have make a deep copy of e.

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

+        let e = e.clone();

         min = Some(match min {
         min = Some(match min {

-            None => e.clone(),
-            Some(n) => e.clone().min(n)
+            None => e,
+            Some(n) => e.min_try1(n)

         });
     }
     min
 }
         });
     }
     min
 }

-// Now, what's happening here? Why do we have to write `clone()`, and why did we not
-// have to write that in our previous version?
+// Now, what's happening here? Why do we have to clone `e`, and why did we not
+// have to do that in our previous version?

 // 
 // The answer is already hidden in the type of `vec_min`: `v` is just borrowed, but
 // the Option<BigInt> that it returns is *owned*. We can't just return one of the elements of `v`,
 // as that would mean that it is no longer in the vector! In our code, this comes up when we update
 // 
 // The answer is already hidden in the type of `vec_min`: `v` is just borrowed, but
 // the Option<BigInt> that it returns is *owned*. We can't just return one of the elements of `v`,
 // as that would mean that it is no longer in the vector! In our code, this comes up when we update

-// the intermediate variable `min`, which also has type `Option<BigInt>`. If you replace `e.clone()`
-// in the `None` arm with `*e`, Rust will complain "Cannot move out of borrowed content". That's because
+// the intermediate variable `min`, which also has type `Option<BigInt>`. If you replace get rid of the
+// `e.clone()`, Rust will complain "Cannot move out of borrowed content". That's because

 // `e` is a `&BigInt`. Assigning `min = Some(*e)` works just like a function call: Ownership of the
 // underlying data is transferred from where `e` borrows from to `min`. But that's not allowed, since
 // we just borrowed `e`, so we cannot empty it! We can, however, call `clone()` on it. Then we own
 // the copy that was created, and hence we can store it in `min`.<br/>
 // `e` is a `&BigInt`. Assigning `min = Some(*e)` works just like a function call: Ownership of the
 // underlying data is transferred from where `e` borrows from to `min`. But that's not allowed, since
 // we just borrowed `e`, so we cannot empty it! We can, however, call `clone()` on it. Then we own
 // the copy that was created, and hence we can store it in `min`.<br/>

-// Of course, making such a full copy is expensive, so we'd like to avoid it. We'll some to that soon.
+// Of course, making such a full copy is expensive, so we'd like to avoid it. We'll some to that in the next part.

 
 // ## `Copy` types
 // But before we go there, I should answer the second question I brought up above: Why did our old `vec_min` work?
 // We stored the minimal `i32` locally without cloning, and Rust did not complain. That's because there isn't
 // really much of an "ownership" when it comes to types like `i32` or `bool`: If you move the value from one
 
 // ## `Copy` types
 // But before we go there, I should answer the second question I brought up above: Why did our old `vec_min` work?
 // We stored the minimal `i32` locally without cloning, and Rust did not complain. That's because there isn't
 // really much of an "ownership" when it comes to types like `i32` or `bool`: If you move the value from one

-// place to another, then both instance are "complete". We also say the value has been *duplicated*. This is in
+// place to another, then both instances are "complete". We also say the value has been *duplicated*. This is in

 // stark contrast to types like `Vec<i32>`, where moving the value results in both the old and the new vector to
 // stark contrast to types like `Vec<i32>`, where moving the value results in both the old and the new vector to

-// point to the same underlying buffer. We don't have two vectors, there's no duplication.
+// point to the same underlying buffer. We don't have two vectors, there's no proper duplication.

 //
 //

-// Rust calls types that can be freely duplicated `Copy` types. `Copy` is another trait, and it
-// is implemented for types like `i32` and `bool`. Remember how we defined the trait `Minimum` by writing
-// `trait Minimum : Copy { ...`? This tells Rust that every type that implements `Minimum` must also
-// implement `Copy`, and that's why the compiler accepted our generic `vec_min` in part 02.
-// `Copy` is the first *marker trait* that we encounter: It does not provide any methods, but
-// makes a promise about the behavior of the type - in this case, being duplicable.
+// Rust calls types that can be easily duplicated `Copy` types. `Copy` is another trait, and it is implemented for
+// types like `i32` and `bool`. Remember how we defined the trait `Minimum` by writing `trait Minimum : Copy { ...`?
+// This tells Rust that every type that implements `Minimum` must also implement `Copy`, and that's why the compiler
+// accepted our generic `vec_min` in part 02. `Copy` is the first *marker trait* that we encounter: It does not provide
+// any methods, but makes a promise about the behavior of the type - in this case, being duplicable.

 
 // If you try to implement `Copy` for `BigInt`, you will notice that Rust
 // does not let you do that. A type can only be `Copy` if all its elements
 // are `Copy`, and that's not the case for `BigInt`. However, we can make
 // `SomethingOrNothing<T>` copy if `T` is `Copy`.
 use part02::{SomethingOrNothing,Something,Nothing};
 
 // If you try to implement `Copy` for `BigInt`, you will notice that Rust
 // does not let you do that. A type can only be `Copy` if all its elements
 // are `Copy`, and that's not the case for `BigInt`. However, we can make
 // `SomethingOrNothing<T>` copy if `T` is `Copy`.
 use part02::{SomethingOrNothing,Something,Nothing};

-impl<T: Copy> Copy for SomethingOrNothing<T>{}
+impl<T: Copy> Copy for SomethingOrNothing<T> {}

 // Again, Rust can generate implementations of `Copy` automatically. If
 // you add `#[derive(Copy,Clone)]` right before the definition of `SomethingOrNothing`,
 // both `Copy` and `Clone` will automatically be implemented.
 
 // ## An operational perspective
 // Instead of looking at what happens "at the surface" (i.e., visible in Rust), one can also explain
 // Again, Rust can generate implementations of `Copy` automatically. If
 // you add `#[derive(Copy,Clone)]` right before the definition of `SomethingOrNothing`,
 // both `Copy` and `Clone` will automatically be implemented.
 
 // ## An operational perspective
 // Instead of looking at what happens "at the surface" (i.e., visible in Rust), one can also explain

-// ownership passing and how `Copy` and `Clone` fit by looking at what happens on the machine.<br/>
+// ownership passing and how `Copy` and `Clone` fit in by looking at what happens on the machine.<br/>

 // When Rust code is executed, passing a value (like `i32` or `Vec<i32>`) to a function will always
 // result in a shallow copy being performed: Rust just copies the bytes representing that value, and
 // considers itself done. That's just like the default copy constructor in C++. Rust, however, will
 // consider this a destructive operation: After copying the bytes elsewhere, the original value must
 // When Rust code is executed, passing a value (like `i32` or `Vec<i32>`) to a function will always
 // result in a shallow copy being performed: Rust just copies the bytes representing that value, and
 // considers itself done. That's just like the default copy constructor in C++. Rust, however, will
 // consider this a destructive operation: After copying the bytes elsewhere, the original value must

-// no longer be used. After all, the two could not share a pointer! If, however, you mark a type `Copy`,
+// no longer be used. After all, the two could now share a pointer! If, however, you mark a type `Copy`,

 // then Rust will *not* consider a move destructive, and just like in C++, the old and new value
 // then Rust will *not* consider a move destructive, and just like in C++, the old and new value

-// can happily coexist. Now, Rust does not allow to to overload the copy constructor. This means that
+// can happily coexist. Now, Rust does not allow you to overload the copy constructor. This means that

 // passing a value around will always be a fast operation, no allocation or any other kind of heap access
 // will happen. In the situations where you would write a copy constructor in C++ (and hence
 // incur a hidden cost on every copy of this type), you'd have the type *not* implement `Copy`, but only
 // `Clone`. This makes the cost explicit.
 
 // ## Lifetimes
 // passing a value around will always be a fast operation, no allocation or any other kind of heap access
 // will happen. In the situations where you would write a copy constructor in C++ (and hence
 // incur a hidden cost on every copy of this type), you'd have the type *not* implement `Copy`, but only
 // `Clone`. This makes the cost explicit.
 
 // ## Lifetimes

-// To fix the performance problems of `vec_min`, we need ti avoid using `clone()`. We'd like
+// To fix the performance problems of `vec_min`, we need to avoid using `clone()`. We'd like

 // the return value to not be owned (remember that this was the source of our need for cloning), but *borrowed*.
 
 // the return value to not be owned (remember that this was the source of our need for cloning), but *borrowed*.
 

-// This is demonstrated by the function `head` that borrows the first element of a vector if it is non-empty.
+// The function `head` demonstrates how that could work: It borrows the first element of a vector if it is non-empty.

 // The type of the function says that it will either return nothing, or it will return a borrowed `T`.
 // We can then borrow the first element of `v` and use it to construct the return value.
 fn head<T>(v: &Vec<T>) -> Option<&T> {
 // The type of the function says that it will either return nothing, or it will return a borrowed `T`.
 // We can then borrow the first element of `v` and use it to construct the return value.
 fn head<T>(v: &Vec<T>) -> Option<&T> {


@@ -104,10 +104,8 @@ fn head<T>(v: &Vec<T>) -> Option<&T> {
         None
     }
 }
         None
     }
 }

-
-// Now, coming back to `head` - here, we are returning a pointer to the first element. But doesn't
-// that mean that callers have to be careful? Imagine `head` would be a C++ function, and we would
-// write the following code.
+// Technically, we are returning a pointer to the first element. But doesn't that mean that callers have to be
+// careful? Imagine `head` would be a C++ function, and we would write the following code.

 /*
   int foo(std::vector<int> v) {
     int *first = head(v);
 /*
   int foo(std::vector<int> v) {
     int *first = head(v);


@@ -115,8 +113,9 @@ fn head<T>(v: &Vec<T>) -> Option<&T> {
     return *first;
   }
 */
     return *first;
   }
 */

-// This is very much like our very first motivating example for ownership, at the beginning of part 04.
-// But this time, the bug is hidden behind the call to `head`. How does Rust solve this? If we translate
+// This is very much like our very first motivating example for ownership, at the beginning of part 04:
+// `push_back` could reallocate the buffer, making `first` an invalid pointer. Again, we have aliasing (of `first`
+// and `v`) and mutation. But this time, the bug is hidden behind the call to `head`. How does Rust solve this? If we translate

 // the code above to Rust, it doesn't compile, so clearly we are good - but how and why?
 // (Notice that have to explicitly assert using `unwrap` that `first` is not `None`, whereas the C++ code
 // above would silently dereference a `NULL`-pointer. But that's another point.)
 // the code above to Rust, it doesn't compile, so clearly we are good - but how and why?
 // (Notice that have to explicitly assert using `unwrap` that `first` is not `None`, whereas the C++ code
 // above would silently dereference a `NULL`-pointer. But that's another point.)


@@ -128,11 +127,11 @@ fn rust_foo(mut v: Vec<i32>) -> i32 {
 
 // To give the answer to this question, we have to talk about the *lifetime* of a borrow. The point is, saying that
 // you borrowed your friend a `Vec<i32>`, or a book, is not good enough, unless you also agree on *how long*
 
 // To give the answer to this question, we have to talk about the *lifetime* of a borrow. The point is, saying that
 // you borrowed your friend a `Vec<i32>`, or a book, is not good enough, unless you also agree on *how long*

-// your friend can borrow. After all, you need to know when you can rely on owning your data (or book) again.
+// your friend can borrow it. After all, you need to know when you can rely on owning your data (or book) again.

 // 
 // 

-// Every borrow in Rust has an associated lifetime. The full type of `head` reads as follows:
-// `fn<'a, T>(&'a Vec<T>) -> Option<&'a T>`. Here, `'a` is a *lifetime variable*, which represents how long the vector has
-// been borrowed. The function type expresses that argument and return value have *the same lifetime*.
+// Every borrow in Rust has an associated lifetime, written `&'a T` for a borrow of type `T` with lifetime `'a`. The full
+// type of `head` reads as follows: `fn<'a, T>(&'a Vec<T>) -> Option<&'a T>`. Here, `'a` is a *lifetime variable*, which
+// represents how long the vector has been borrowed. The function type expresses that argument and return value have *the same lifetime*.

 // 
 // When analyzing the code of `rust_foo`, Rust has to assign a lifetime to `first`. It will choose the scope
 // where `first` is valid, which is the entire rest of the function. Because `head` ties the lifetime of its
 // 
 // When analyzing the code of `rust_foo`, Rust has to assign a lifetime to `first`. It will choose the scope
 // where `first` is valid, which is the entire rest of the function. Because `head` ties the lifetime of its


@@ -147,4 +146,4 @@ fn rust_foo(mut v: Vec<i32>) -> i32 {
 // Most of the time, we don't have to explicitly add lifetimes to function types. This is thanks to *lifetimes elision*,
 // where Rust will automatically insert lifetimes we did not specify, following some [simple, well-documented rules](http://doc.rust-lang.org/stable/book/lifetimes.html#lifetime-elision).
 
 // Most of the time, we don't have to explicitly add lifetimes to function types. This is thanks to *lifetimes elision*,
 // where Rust will automatically insert lifetimes we did not specify, following some [simple, well-documented rules](http://doc.rust-lang.org/stable/book/lifetimes.html#lifetime-elision).
 

-// [index](main.html) | [previous](part05.html) | [next](main.html)
+// [index](main.html) | [previous](part05.html) | [next](part07.html)