Merge pull request #41 from bzindovic/bzindovic-part13-clarity-fix

[rust-101.git] / src / part05.rs

diff --git a/src/part05.rs b/src/part05.rs
index 72c787d00c7ef7c518f4fb1b7e5ff700b8f8eed0..7ad87545a92c80a5b21ef507b2bd26f9e40c3fab 100644 (file)
--- a/src/part05.rs
+++ b/src/part05.rs
@@ -2,26 +2,28 @@
 // ========================
 
 // ## Big Numbers
 // ========================
 
 // ## Big Numbers

-//@ In the course of the next few parts, we are going to build a data-structure for computations with
-//@ *big* numbers. We would like to not have an upper bound to how large these numbers can get, with
-//@ the memory of the machine being the only limit.
-//@ 
-//@ We start by deciding how to represent such big numbers. One possibility here is
-//@ to use a vector "digits" of the number. This is like "1337" being a vector of four digits (1, 3, 3, 7),
-//@ except that we will use `u64` as type of our digits, meaning we have 2^64 individual digits. Now we just
-//@ have to decide the order in which we store numbers. I decided that we will store the least significant
-//@ digit first. This means that "1337" would actually become (7, 3, 3, 1).<br/>
+
+//@ In the course of the next few parts, we are going to build a data-structure for computations
+//@ with *big* numbers. We would like to not have an upper bound to how large these numbers can
+//@ get, with the memory of the machine being the only limit.
+//@
+//@ We start by deciding how to represent such big numbers. One possibility here is to use a vector
+//@ "digits" of the number. This is like "1337" being a vector of four digits (1, 3, 3, 7), except
+//@ that we will use `u64` as type of our digits, meaning we have 2^64 individual digits. Now we
+//@ just have to decide the order in which we store numbers. I decided that we will store the least
+//@ significant digit first. This means that "1337" would actually become (7, 3, 3, 1). <br/>

 //@ Finally, we declare that there must not be any trailing zeros (corresponding to
 //@ useless leading zeros in our usual way of writing numbers). This is to ensure that
 //@ the same number can only be stored in one way.
 
 //@ To write this down in Rust, we use a `struct`, which is a lot like structs in C:
 //@ Finally, we declare that there must not be any trailing zeros (corresponding to
 //@ useless leading zeros in our usual way of writing numbers). This is to ensure that
 //@ the same number can only be stored in one way.
 
 //@ To write this down in Rust, we use a `struct`, which is a lot like structs in C:

-//@ Just a bunch of named fields. Every field can be private to the current module (which is the default),
-//@ or public (which is indicated by a `pub` in front of the name). For the sake of the tutorial, we make
-//@ `data` public - otherwise, the next parts of this course could not work on `BigInt`s. Of course, in a
-//@ real program, one would make the field private to ensure that the invariant (no trailing zeros) is maintained.
+//@ Just a bunch of named fields. Every field can be private to the current module (which is the
+//@ default), or public (which is indicated by a `pub` in front of the name). For the sake of the
+//@ tutorial, we make `data` public - otherwise, the next parts of this course could not work on
+//@ `BigInt`s. Of course, in a real program, one would make the field private to ensure that the
+//@ invariant (no trailing zeros) is maintained.

 pub struct BigInt {
 pub struct BigInt {

-    pub data: Vec<u64>,
+    pub data: Vec<u64>, // least significant digit first, no trailing zeros

 }
 
 // Now that we fixed the data representation, we can start implementing methods on it.
 }
 
 // Now that we fixed the data representation, we can start implementing methods on it.


@@ -31,7 +33,7 @@ impl BigInt {
     //@ fields and initial values assigned to them.
     pub fn new(x: u64) -> Self {
         if x == 0 {
     //@ fields and initial values assigned to them.
     pub fn new(x: u64) -> Self {
         if x == 0 {

-            BigInt { data: vec![] }
+            BigInt { data: vec![] }                                 /*@*/

         } else {
             BigInt { data: vec![x] }                                /*@*/
         }
         } else {
             BigInt { data: vec![x] }                                /*@*/
         }


@@ -47,30 +49,32 @@ impl BigInt {
         }
     }
 
         }
     }
 

-    // We can convert any vector of digits into a number, by removing trailing zeros. The `mut`
-    // declaration for `v` here is just like the one in `let mut ...`, it says that we will locally
-    // change the vector `v`.
+    // We can convert any little-endian vector of digits (i.e., least-significant digit first) into
+    // a number, by removing trailing zeros. The `mut` declaration for `v` here is just like the
+    // one in `let mut ...`: We completely own `v`, but Rust still asks us to make our intention of
+    // modifying it explicit. This `mut` is *not* part of the type of `from_vec` - the caller has
+    // to give up ownership of `v` anyway, so they don't care anymore what you do to it.

     // 
     // **Exercise 05.1**: Implement this function.
     // 
     // 
     // **Exercise 05.1**: Implement this function.
     // 

-    // *Hint*: You can use `pop()` to remove the last element of a vector.
+    // *Hint*: You can use `pop` to remove the last element of a vector.

     pub fn from_vec(mut v: Vec<u64>) -> Self {
         unimplemented!()
     }
 }
 
 // ## Cloning
     pub fn from_vec(mut v: Vec<u64>) -> Self {
         unimplemented!()
     }
 }
 
 // ## Cloning

-//@ If you have a close look at the type of `BigInt::from_vec`, you will notice that it
-//@ consumes the vector `v`. The caller hence loses access to its vector. There is however something
-//@ we can do if we don't want that to happen: We can explicitly `clone` the vector,
-//@ which means that a full (or *deep*) copy will be performed. Technically,
-//@ `clone` takes a borrowed vector, and returns a fully owned one.
+//@ If you take a close look at the type of `BigInt::from_vec`, you will notice that it consumes
+//@ the vector `v`. The caller hence loses access to its vector. However, there is something we can
+//@ do if we don't want that to happen: We can explicitly `clone` the vector, which means that a
+//@ full (or *deep*) copy will be performed. Technically, `clone` takes a borrowed vector in the
+//@ form of a shared reference, and returns a fully owned one.

 fn clone_demo() {
     let v = vec![0,1 << 16];
     let b1 = BigInt::from_vec((&v).clone());
     let b2 = BigInt::from_vec(v);
 }
 fn clone_demo() {
     let v = vec![0,1 << 16];
     let b1 = BigInt::from_vec((&v).clone());
     let b2 = BigInt::from_vec(v);
 }

-//@ Rust has special treatment for methods that borrow its `self` argument (like `clone`, or
+//@ Rust has special treatment for methods that borrow their `self` argument (like `clone`, or

 //@ like `test_invariant` above): It is not necessary to explicitly borrow the receiver of the
 //@ method. Hence you could replace `(&v).clone()` by `v.clone()` above. Just try it!
 
 //@ like `test_invariant` above): It is not necessary to explicitly borrow the receiver of the
 //@ method. Hence you could replace `(&v).clone()` by `v.clone()` above. Just try it!
 


@@ -88,16 +92,16 @@ impl Clone for BigInt {
 //@ These `#[...]` annotations at types (and functions, modules, crates) are called *attributes*.
 //@ We will see some more examples of attributes later.
 
 //@ These `#[...]` annotations at types (and functions, modules, crates) are called *attributes*.
 //@ We will see some more examples of attributes later.
 

-// We can also make the type `SomethingOrNothing<T>` implement `Clone`. 
-//@ However, that can only work if `T` is `Clone`! So we have to add this bound to `T` when we introduce
-//@ the type variable.
+// We can also make the type `SomethingOrNothing<T>` implement `Clone`.
+//@ However, that can only work if `T` is `Clone`! So we have to add this bound to `T` when we
+//@ introduce the type variable.

 use part02::{SomethingOrNothing,Something,Nothing};
 impl<T: Clone> Clone for SomethingOrNothing<T> {
     fn clone(&self) -> Self {
         match *self {                                               /*@*/
             Nothing => Nothing,                                     /*@*/
             //@ In the second arm of the match, we need to talk about the value `v`
 use part02::{SomethingOrNothing,Something,Nothing};
 impl<T: Clone> Clone for SomethingOrNothing<T> {
     fn clone(&self) -> Self {
         match *self {                                               /*@*/
             Nothing => Nothing,                                     /*@*/
             //@ In the second arm of the match, we need to talk about the value `v`

-            //@ that's stored in `self`. However, if we would write the pattern as
+            //@ that's stored in `self`. However, if we were to write the pattern as

             //@ `Something(v)`, that would indicate that we *own* `v` in the code
             //@ after the arrow. That can't work though, we have to leave `v` owned by
             //@ whoever called us - after all, we don't even own `self`, we just borrowed it.
             //@ `Something(v)`, that would indicate that we *own* `v` in the code
             //@ after the arrow. That can't work though, we have to leave `v` owned by
             //@ whoever called us - after all, we don't even own `self`, we just borrowed it.


@@ -110,39 +114,43 @@ impl<T: Clone> Clone for SomethingOrNothing<T> {
 //@ Again, Rust will generate this implementation automatically if you add
 //@ `#[derive(Clone)]` right before the definition of `SomethingOrNothing`.
 
 //@ Again, Rust will generate this implementation automatically if you add
 //@ `#[derive(Clone)]` right before the definition of `SomethingOrNothing`.
 

-// **Exercise 05.2**: Write some more functions on `BigInt`. What about a function that returns the number of
-// digits? The number of non-zero digits? The smallest/largest digit?
+// **Exercise 05.2**: Write some more functions on `BigInt`. What about a function that returns the
+// number of digits? The number of non-zero digits? The smallest/largest digit? Of course, these
+// should all take `self` as a shared reference (i.e., in borrowed form).

 
 // ## Mutation + aliasing considered harmful (part 2)
 
 // ## Mutation + aliasing considered harmful (part 2)

-//@ Now that we know how to borrow a part of an `enum` (like `v` above), there's another example for why we
-//@ have to rule out mutation in the presence of aliasing. First, we define an `enum` that can hold either
-//@ a number, or a string.
+//@ Now that we know how to create references to contents of an `enum` (like `v` above), there's
+//@ another example we can look at for why we have to rule out mutation in the presence of
+//@ aliasing. First, we define an `enum` that can hold either a number, or a string.

 enum Variant {
     Number(i32),
     Text(String),
 }
 enum Variant {
     Number(i32),
     Text(String),
 }

-//@ Now consider the following piece of code. Like above, `n` will be a borrow of a part of `var`,
-//@ and since we wrote `ref mut`, the borrow will be mutable. In other words, right after the match, `ptr`
-//@ points to the number that's stored in `var`, where `var` is a `Number`. Remember that `_` means
-//@ "we don't care".
+//@ Now consider the following piece of code. Like above, `n` will be a reference to a part of
+//@ `var`, and since we wrote `ref mut`, the reference will be unique and mutable. In other words,
+//@ right after the match, `ptr` points to the number that's stored in `var`, where `var` is a
+//@ `Number`. Remember that `_` means "we don't care".

 fn work_on_variant(mut var: Variant, text: String) {
     let mut ptr: &mut i32;
     match var {
         Variant::Number(ref mut n) => ptr = n,
         Variant::Text(_) => return,
     }
 fn work_on_variant(mut var: Variant, text: String) {
     let mut ptr: &mut i32;
     match var {
         Variant::Number(ref mut n) => ptr = n,
         Variant::Text(_) => return,
     }

-    /* var = Variant::Text(text); */
+    /* var = Variant::Text(text); */                                /* BAD! */

     *ptr = 1337;
 }
     *ptr = 1337;
 }

-//@ Now, imagine what would happen if we were permitted to also mutate `var`. We could, for example,
-//@ make it a `Text`. However, `ptr` still points to the old location! Hence `ptr` now points somewhere
-//@ into the representation of a `String`. By changing `ptr`, we manipulate the string in completely
-//@ unpredictable ways, and anything could happen if we were to use it again! (Technically, the first field
-//@ of a `String` is a pointer to its character data, so by overwriting that pointer with an integer,
-//@ we make it a completely invalid address. When the destructor of `var` runs, it would try to deallocate
-//@ that address, and Rust would eat your laundry - or whatever.)
+//@ Now, imagine what would happen if we were permitted to also mutate `var`. We could, for
+//@ example, make it a `Text`. However, `ptr` still points to the old location! Hence `ptr` now
+//@ points somewhere into the representation of a `String`. By changing `ptr`, we manipulate the
+//@ string in completely unpredictable ways, and anything could happen if we were to use it again!
+//@ (Technically, the first field of a `String` is a pointer to its character data, so by
+//@ overwriting that pointer with an integer, we make it a completely invalid address. When the
+//@ destructor of `var` runs, it would try to deallocate that address, and Rust would eat your
+//@ laundry - or whatever.)

 //@ 
 //@ 

-//@ I hope this example clarifies why Rust has to rule out mutation in the presence of aliasing *in general*,
-//@ not just for the specific case of a buffer being reallocated, and old pointers becoming hence invalid.
+//@ I hope this example clarifies why Rust has to rule out mutation in the presence of aliasing
+//@ *in general*, not just for the specific case of a buffer being reallocated, and old pointers
+//@ becoming hence invalid.

 
 

-//@ [index](main.html) | [previous](part04.html) | [next](part06.html)
+//@ [index](main.html) | [previous](part04.html) | [raw source](workspace/src/part05.rs) |
+//@ [next](part06.html)