Merge pull request #28 from NanXiao/patch-1

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

diff --git a/src/part06.rs b/src/part06.rs
index 5b90142825798a25321352f136b1c9c913d555a7..7113094f457e3a77d77267ba129ae85bab165a46 100644 (file)
--- a/src/part06.rs
+++ b/src/part06.rs
@@ -26,11 +26,11 @@ 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)                                /*@*/


@@ -38,19 +38,19 @@ 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
+//@ 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
 //@ 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 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
+//@ 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/>
 //@ 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.
+//@ 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
 //@ But before we go there, I should answer the second question I brought up above: Why did our old `vec_min` work?
 
 // ## `Copy` types
 //@ But before we go there, I should answer the second question I brought up above: Why did our old `vec_min` work?


@@ -94,10 +94,11 @@ impl<T: Copy> Copy for SomethingOrNothing<T> {}
 // ## 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*.
 // ## 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*.

+//@ 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 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`.
 //@ 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.
+//@ 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])                                                 /*@*/


@@ -126,25 +127,25 @@ fn rust_foo(mut v: Vec<i32>) -> i32 {
     *first.unwrap()
 }
 
     *first.unwrap()
 }
 

-//@ To give the answer to this question, we have to talk about the *lifetime* of a borrow. The point is, saying that
+//@ 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.
 //@ 
 //@ 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
+//@ 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
 //@ 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*.
+//@ 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
 //@ 
 //@ 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.
+//@ 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
+//@ 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.
 //@ 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).
+//@ 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)