Merge pull request #34 from louy2/patch-4

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

diff --git a/src/part06.rs b/src/part06.rs
index 35cd5a9dc0a82b8a35daf9618ad7498dca0d6937..21fe644758fc7813a7a7cae1b53698005bbcb1b2 100644 (file)
--- a/src/part06.rs
+++ b/src/part06.rs
@@ -8,9 +8,10 @@ use part05::BigInt;
 // that computes the minimum of a list of `BigInt`. First, we have to write `min` for `BigInt`.
 impl BigInt {
     fn min_try1(self, other: Self) -> Self {
 // that computes the minimum of a list of `BigInt`. First, we have to write `min` for `BigInt`.
 impl BigInt {
     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`.
+        //@ 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());
         // 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.
         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.


@@ -26,11 +27,12 @@ impl BigInt {
 }
 
 // Now we can write `vec_min`.
 }
 
 // Now we can write `vec_min`.

-//@ However, in order to make it type-check, we have to make a full (deep) copy of e by calling `clone()`.

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

+    // If `v` is a shared reference to a vector, then the default for iterating over it is to call
+    // `iter`, the iterator that borrows the elements.

     for e in v {
     for e in v {

-        let e = e.clone();                                          /*@*/
+        let e = e.clone();

         min = Some(match min {                                      /*@*/
             None => e,                                              /*@*/
             Some(n) => e.min_try1(n)                                /*@*/
         min = Some(match min {                                      /*@*/
             None => e,                                              /*@*/
             Some(n) => e.min_try1(n)                                /*@*/


@@ -38,38 +40,40 @@ fn vec_min(v: &Vec<BigInt>) -> Option<BigInt> {
     }
     min
 }
     }
     min
 }

-//@ 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?
+//@ Now, what's happening here? Why do we have to to make a full (deep) copy of `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 get rid of the
+//@ the intermediate variable `min`, which also has type `Option<BigInt>`. If you 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
 //@ `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/>
-//@ 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.
+//@ underlying data is transferred from `e` 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 come to that
+//@ in the next part.

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

 //@
 //@

-//@ 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.
+//@ 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`.
+//@ 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> {}
 //@ Again, Rust can generate implementations of `Copy` automatically. If
 use part02::{SomethingOrNothing,Something,Nothing};
 impl<T: Copy> Copy for SomethingOrNothing<T> {}
 //@ Again, Rust can generate implementations of `Copy` automatically. If


@@ -78,26 +82,29 @@ impl<T: Copy> Copy for SomethingOrNothing<T> {}
 
 //@ ## An operational perspective
 //@ Instead of looking at what happens "at the surface" (i.e., visible in Rust), one can also explain
 
 //@ ## 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 in 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 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
-//@ 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.
+//@ 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 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
 
 // ## Lifetimes

-//@ 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*.
+//@ 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*. In other words, we want to return a shared reference to the minimal element.

 
 

-//@ 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.
+//@ The function `head` demonstrates how that could work: It returns a reference to 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 obtain a reference to the first
+//@ element of `v` and use it to construct the return value.

 fn head<T>(v: &Vec<T>) -> Option<&T> {
     if v.len() > 0 {
         Some(&v[0])                                                 /*@*/
 fn head<T>(v: &Vec<T>) -> Option<&T> {
     if v.len() > 0 {
         Some(&v[0])                                                 /*@*/


@@ -105,8 +112,9 @@ fn head<T>(v: &Vec<T>) -> Option<&T> {
         None
     }
 }
         None
     }
 }

-// 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.
+// 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);


@@ -114,37 +122,47 @@ 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:
-//@ `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.)
+//@ 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.)

 fn rust_foo(mut v: Vec<i32>) -> i32 {
     let first: Option<&i32> = head(&v);
     /* v.push(42); */
     *first.unwrap()
 }
 
 fn rust_foo(mut v: Vec<i32>) -> i32 {
     let first: Option<&i32> = head(&v);
     /* v.push(42); */
     *first.unwrap()
 }
 

-//@ 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 it. After all, you need to know when you can rely on owning your data (or book) again.
+//@ To give the answer to this question, we have to talk about the *lifetime* of a reference. 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 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, 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*.
+//@ Every reference in Rust has an associated lifetime, written `&'a T` for a reference with
+//@ lifetime `'a` to something of type `T`. 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 for 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
-//@ argument and return value together, this means that `&v` also has to borrow `v` for the entire duration of
-//@ the function. So when we try to borrow `v` mutable for `push`, Rust complains that the two borrows (the one
-//@ for `head`, and the one for `push`) overlap. Lucky us! Rust caught our mistake and made sure we don't crash the program.
+//@ 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 argument and return value together, this means that `&v` also has to borrow
+//@ `v` for the entire duration of the function `rust_foo`. So when we try to create a unique
+//@ reference to `v` for `push`, Rust complains that the two references (the one for `head`, and
+//@ the one for `push`) overlap, so neither of them can be unique. Lucky us! Rust caught our
+//@ mistake and made sure we don't crash the program.

 //@ 
 //@ 

-//@ So, to sum this up: Lifetimes enable Rust to reason about *how long* a pointer has been borrowed. We can thus
-//@ safely write functions like `head`, that return pointers into data they got as argument, and make sure they
-//@ are used correctly, *while looking only at the function type*. At no point in our analysis of `rust_foo` did
-//@ we have to look *into* `head`. That's, of course, crucial if we want to separate library code from application code.
-//@ 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).
+//@ So, to sum this up: Lifetimes enable Rust to reason about *how long* a reference is valid, how
+//@ long ownership has been borrowed. We can thus safely write functions like `head`, that return
+//@ references into data they got as argument, and make sure they are used correctly, *while
+//@ looking only at the function type*. At no point in our analysis of `rust_foo` did we have to
+//@ look *into* `head`. That's, of course, crucial if we want to separate library code from
+//@ application code. Most of the time, we don't have to explicitly add lifetimes to function
+//@ types. This is thanks to *lifetime elision*, where Rust will automatically insert lifetimes we
+//@ did not specify, following some simple, well-documented
+//@ [rules](https://doc.rust- lang.org/stable/book/lifetimes.html#lifetime-elision).

 
 

-//@ [index](main.html) | [previous](part05.html) | [next](part07.html)
+//@ [index](main.html) | [previous](part05.html) | [raw source](workspace/src/part06.rs) |
+//@ [next](part07.html)