Merge pull request #4 from wimh/PratialEq

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

diff --git a/src/part05.rs b/src/part05.rs
index 25c98e2b52595342b099cab9961861c43d4de560..6780ca306002e08a8b01312af83ecc7494dd4d4e 100644 (file)
--- a/src/part05.rs
+++ b/src/part05.rs
@@ -1,137 +1,150 @@
-// Rust-101, Part 05: Copy, Clone
-// ==============================
+// Rust-101, Part 05: Clone
+// ========================

 
 

-use std::cmp;
-use std::ops;
+// ## 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/>
+//@ 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.

 
 

-// In the course of the next few parts, we are going to build a data-structure for
-// computations with *bug* 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 of "small" numbers, which we will then consider the "digits"
-// of the big number. This is like "1337" being a vector of 4 small numbers (1, 3, 3, 7),
-// except that we will use `u64` as type of our base numbers. 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:
-// Just a collection of a bunch of named fields. Every field can be private to the current module
-// (which is the default), or public (which would be indicated by a `pub` in front of the name).
-// For the sake of the tutorial, we make `dat` 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.
+//@ 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.

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

-    // Let's start with a constructor, creating a `BigInt` from an ordinary integer.
-    // To create an instance of a struct, we write its name followed by a list of
-    // fields and initial values assigned to them.
+    //@ Let's start with a constructor, creating a `BigInt` from an ordinary integer.
+    //@ To create an instance of a struct, we write its name followed by a list of
+    //@ fields and initial values assigned to them.

     pub fn new(x: u64) -> Self {
         if x == 0 {
     pub fn new(x: u64) -> Self {
         if x == 0 {

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

         } else {
         } else {

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

         }
     }
 
         }
     }
 

-    // It can often be useful to encode the invariant of a data-structure in code, so here
-    // is a check that detects useless trailing zeros.
+    //@ It can often be useful to encode the invariant of a data-structure in code, so here
+    //@ is a check that detects useless trailing zeros.

     pub fn test_invariant(&self) -> bool {
         if self.data.len() == 0 {
             true
         } else {
     pub fn test_invariant(&self) -> bool {
         if self.data.len() == 0 {
             true
         } else {

-            self.data[self.data.len() - 1] != 0
+            self.data[self.data.len() - 1] != 0                     /*@*/

         }
     }
 
     // We can convert any vector of digits into a number, by removing trailing zeros. The `mut`
         }
     }
 
     // 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`. In this case, we need to make that annotation to be able to call `pop`
-    // on `v`.
+    // 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.
+    // 
+    // *Hint*: You can use `pop` to remove the last element of a vector.

     pub fn from_vec(mut v: Vec<u64>) -> Self {
     pub fn from_vec(mut v: Vec<u64>) -> Self {

-        while v.len() > 0 && v[v.len()-1] == 0 {
-            v.pop();
-        }
-        BigInt { data: v }
+        unimplemented!()

     }
 }
 
     }
 }
 

-// 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. 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.
+// ## 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.

 fn clone_demo() {
     let v = vec![0,1 << 16];
 fn clone_demo() {
     let v = vec![0,1 << 16];

-    let b1 = BigInt::from_vec(v.clone());
+    let b1 = BigInt::from_vec((&v).clone());

     let b2 = BigInt::from_vec(v);
 }
     let b2 = BigInt::from_vec(v);
 }

+//@ Rust has special treatment for methods that borrow its `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!

 
 

-// To be clonable is a property of a type, and as such, naturally expressed with a trait.
-// In fact, Rust already comes with a trait `Clone` for exactly this purpose. We can hence
-// make our `BigInt` clonable as well.
+//@ To be clonable is a property of a type, and as such, naturally expressed with a trait.
+//@ In fact, Rust already comes with a trait `Clone` for exactly this purpose. We can hence
+//@ make our `BigInt` clonable as well.

 impl Clone for BigInt {
     fn clone(&self) -> Self {
 impl Clone for BigInt {
     fn clone(&self) -> Self {

-        BigInt { data: self.data.clone() }
+        BigInt { data: self.data.clone() }                          /*@*/

     }
 }
     }
 }

-// Making a type clonable is such a common exercise that Rust can even help you doing it:
-// If you add `#[derive(Clone)]' right in front of the definition of `BigInt`, Rust will
-// generate an implementation of `clone` that simply clones all the fields. Try it!
-// 
-// To put this in perspective, `clone` in Rust corresponds to what people usually manually do in
-// the copy constructor of a C++ class: It creates new, independent instance containing the
-// same values. Contrary to that, if you pass something to a function normally (like the
-// second call to `from_vec` in `clone_demo`), only a *shallow* copy is created: The fields
-// are copied, but pointers are simply duplicated. This corresponds to the default copy
-// constructor in C++. Rust assumes that after such a copy, the old value is useless
-// (as the new one uses the same pointers), and hence considers the data semantically
-// moved to the copy. That's another explanation of why Rust does not let you access
-// a vector anymore after you passed ownership to some function.
+//@ Making a type clonable is such a common exercise that Rust can even help you doing it:
+//@ If you add `#[derive(Clone)]` right in front of the definition of `BigInt`, Rust will
+//@ generate an implementation of `Clone` that simply clones all the fields. Try it!
+//@ These `#[...]` annotations at types (and functions, modules, crates) are called *attributes*.
+//@ We will see some more examples of attributes later.

 
 

-// 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!
-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`.
-        // 
-        // If you carefully check the type of `BigInt::test_invariant`, you may be surprised that
-        // we can call the function this way. Doesn't it take `self` in borrowed form? Indeed,
-        // the explicit way to do that would be to call `(&other).test_invariant()`. However, the
-        // `self` argument of a method is treated specially by Rust, and borrowing happens automatically here.
-        debug_assert!(self.test_invariant() && other.test_invariant());
-        // 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**: Fill in this code.
-            panic!("Not yet implemented.");
-        }
+// 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`
+            //@ that's stored in `self`. However, if we would 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.
+            //@ By writing `Something(ref v)`, we borrow `v` for the duration of the match
+            //@ arm. That's good enough for cloning it.
+            Something(ref v) => Something(v.clone()),               /*@*/
+        }                                                           /*@*/

     }
 }
     }
 }

+//@ 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? Of course, these should all just borrow `self`.

 
 

-fn vec_min(v: &Vec<BigInt>) -> Option<BigInt> {
-    let mut min: Option<BigInt> = None;
-    for e in v {
-        // In the loop, `e` now has type `&i32`, so we have to dereference it.
-        min = Some(match min {
-            None => e.clone(),
-            Some(n) => e.clone().min(n)
-        });
+// ## 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.
+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".
+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,

     }
     }

-    min
+    /* var = Variant::Text(text); */                                /* BAD! */
+    *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.)
+//@ 
+//@ 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) | [raw source](https://www.ralfj.de/git/rust-101.git/blob_plain/HEAD:/workspace/src/part05.rs) | [next](part06.html)