tune explanation of PhantomData ownership annotation

[rust-101.git] / src / part02.rs

diff --git a/src/part02.rs b/src/part02.rs
index b8641df5877b32d0116c21e32e765ddd9067561d..bd8abf0dc5657d6378c68c51592de6c1f0f7d18c 100644 (file)
--- a/src/part02.rs
+++ b/src/part02.rs
@@ -1,106 +1,127 @@
 // Rust-101, Part 02: Generic types, Traits
 // ========================================
 
 // Rust-101, Part 02: Generic types, Traits
 // ========================================
 

-use std;
+//@ Let us for a moment reconsider the type `NumberOrNothing`. Isn't it a bit annoying that we
+//@ had to hard-code the type `i32` in there? What if tomorrow, we want a `CharOrNothing`, and
+//@ later a `FloatOrNothing`? Certainly we don't want to re-write the type and all its inherent methods.

 
 

-// Let us for a moment reconsider the type `NumberOrNothing`. Isn't it a bit
-// annoying that we had to hard-code the type `i32` in there? What if tomorrow,
-// we want a `CharOrNothing`, and later a `FloatOrNothing`? Certainly we don't
-// want to re-write the type and all its inherent methods.
+// ## Generic datatypes

 
 

-// The solution to this is called *generics* or *polymorphism* (the latter is Greek,
-// meaning "many shapes"). You may know something similar from C++ (where it's called
-// *templates*) or Java, or one of the many functional languages. So here, we define
-// a generic type `SomethingOrNothing`.
-enum SomethingOrNothing<T>  {
+//@ The solution to this is called *generics* or *polymorphism* (the latter is Greek,
+//@ meaning "many shapes"). You may know something similar from C++ (where it's called
+//@ *templates*) or Java, or one of the many functional languages. So here, we define
+//@ a generic type `SomethingOrNothing`.
+pub enum SomethingOrNothing<T>  {

     Something(T),
     Nothing,
 }
     Something(T),
     Nothing,
 }

-use self::SomethingOrNothing::{Something,Nothing};
-// What this does is to define an entire family of types: We can now write
-// `SomethingOrNothing<i32>` to get back our `NumberOrNothing`, but we
-// can also write `SomethingOrNothing<bool>` or even `SomethingOrNothing<SomethingOrNothing<i32>>`.
-// In fact, such a type is so useful that it is already present in the standard
-// library: It's called an *option type*, written `Option<T>`.
-// Go check out its [documentation](http://doc.rust-lang.org/stable/std/option/index.html)!
-// (And don't worry, there's indeed lots of material mentioned there that we did not cover yet.)
+// Instead of writing out all the variants, we can also just import them all at once.
+pub use self::SomethingOrNothing::*;
+//@ What this does is to define an entire family of types: We can now write
+//@ `SomethingOrNothing<i32>` to get back our `NumberOrNothing`.
+type NumberOrNothing = SomethingOrNothing<i32>;
+//@ However, we can also write `SomethingOrNothing<bool>` or even `SomethingOrNothing<SomethingOrNothing<i32>>`.
+//@ In fact, such a type is so useful that it is already present in the standard library: It's called an
+//@ *option type*, written `Option<T>`. Go check out its [documentation](http://doc.rust-lang.org/stable/std/option/index.html)!
+//@ (And don't worry, there's indeed lots of material mentioned there that we did not cover yet.)

 
 

-// **Exercise**: Write functions converting between `SomethingOrNothing<T>` and `Option<T>`. You will have to use
-// the names of the constructor of `Option`, which you can find in the documentation I linked above.
-// 
-// Here's a skeleton for your solution, you only have to fill in the function bodies.
-// (`panic!` is, again, a macro - this one terminates execution when it is reached).
-// 
-// Notice the syntax for giving generic implementations to generic types: Think of the first `<T>` 
-// as *declaring* a type variable ("I am doing something for all types `T`"), and the second `<T>` as
-// *using* that variable ("The thing I do, is implement `SomethingOrNothing<T>`").
+// ## Generic `impl`, Static functions
+//@ The types are so similar, that we can provide a generic function to construct a `SomethingOrNothing<T>`
+//@ from an `Option<T>`, and vice versa.
+//@ 
+//@ Notice the syntax for giving generic implementations to generic types: Think of the first `<T>` 
+//@ as *declaring* a type variable ("I am doing something for all types `T`"), and the second `<T>` as
+//@ *using* that variable ("The thing I do, is implement `SomethingOrNothing<T>`").
+//@
+// Inside an `impl`, `Self` refers to the type we are implementing things for. Here, it is
+// an alias for `SomethingOrNothing<T>`.
+//@ Remember that `self` is the `this` of Rust, and implicitly has type `Self`.

 impl<T> SomethingOrNothing<T> {
     fn new(o: Option<T>) -> Self {
 impl<T> SomethingOrNothing<T> {
     fn new(o: Option<T>) -> Self {

-        panic!("Not yet implemented.")
+        match o { None => Nothing, Some(t) => Something(t) }        /*@*/

     }
 
     fn to_option(self) -> Option<T> {
     }
 
     fn to_option(self) -> Option<T> {

-        panic!("Not yet implemented.")
+        match self { Nothing => None, Something(t) => Some(t) }     /*@*/

     }
 }
     }
 }

-// Inside an `impl`, `Self` refers to the type we are implementing things for. Here, it is
-// an alias for `SomethingOrNothing<T>`.
-// Remember that `self` is the `this` of Rust, and implicitly has type `Self`.
-// 
-// Observe how `new` does *not* have a `self` parameter. This corresponds to a `static` method
-// in Java or C++. In fact, `new` is the Rust convention for defining constructors: They are
-// nothing special, just static functions returning `Self`.
-
-// You can call static functions, and in particular constructors, as follows:
+//@ Observe how `new` does *not* have a `self` parameter. This corresponds to a `static` method
+//@ in Java or C++. In fact, `new` is the Rust convention for defining constructors: They are
+//@ nothing special, just static functions returning `Self`.
+//@ 
+// You can call static functions, and in particular constructors, as demonstrated in `call_constructor`.

 fn call_constructor(x: i32) -> SomethingOrNothing<i32> {
     SomethingOrNothing::new(Some(x))
 }
 
 fn call_constructor(x: i32) -> SomethingOrNothing<i32> {
     SomethingOrNothing::new(Some(x))
 }
 

-// Now that we have a generic `SomethingOrNothing`, wouldn't it be nice to also gave a generic
-// `vec_min`? Of course, we can't take the minimum of a vector of *any* type. It has to be a type
-// supporting a `min` operation. Rust calls such properties that we may demand of types *traits*.
+// ## Traits
+//@ Now that we have a generic `SomethingOrNothing`, wouldn't it be nice to also gave a generic
+//@ `vec_min`? Of course, we can't take the minimum of a vector of *any* type. It has to be a type
+//@ supporting a `min` operation. Rust calls such properties that we may demand of types *traits*.

 
 

-// So, as a first step towards a generic `vec_min`, we define a `Minimum` trait.
-// For now, just ignore the `Copy`, we will come back to this point later.
-// A `trait` is a lot like interfaces in Java: You define a bunch of functions
-// you want to have implemented, and their argument and return types.
-trait Minimum : Copy {
-    fn min(a: Self, b: Self) -> Self;
+//@ So, as a first step towards a generic `vec_min`, we define a `Minimum` trait.
+//@ For now, just ignore the `Copy`, we will come back to this point later.
+//@ A `trait` is a lot like interfaces in Java: You define a bunch of functions
+//@ you want to have implemented, and their argument and return types. <br/>
+//@ The function `min` takes to arguments of the same type, but I made the
+//@ first argument the special `self` argument. I could, alternatively, have
+//@ made `min` a static function as follows: `fn min(a: Self, b: Self) -> Self`.
+//@ However, in Rust one typically prefers methods over static function wherever possible.
+pub trait Minimum : Copy {
+    fn min(self, b: Self) -> Self;

 }
 
 }
 

-// Now we can write `vec_min`, generic over a type `T` that we demand to satisfy the `Minimum` trait.
-// This is called a *trait bound*.
-// The only difference to the version from the previous part is that we call `T::min` (the `min`
-// function provided for type `T`) instead of `std::cmp::min`.
-// 
-// Notice a crucial difference to templates in C++: We actually have to declare which traits
-// we want the type to satisfy. If we left away the `Minimum`, Rust would have complained that
-// we cannot call `min`. Just try it! There is no reason to believe that `T` provides such an operation.
-// This is in strong contrast to C++, where the compiler only checks such details when the
-// function is actually used.
-fn vec_min<T: Minimum>(v: Vec<T>) -> SomethingOrNothing<T> {
+//@ Next, we write `vec_min` as a generic function over a type `T` that we demand to satisfy the `Minimum` trait.
+//@ This requirement is called a *trait bound*.
+//@ The only difference to the version from the previous part is that we call `e.min(n)` instead
+//@ of `std::cmp::min(n, e)`. Rust automatically figures out that `n` is of type `T`, which implements
+//@ the `Minimum` trait, and hence we can call that function.
+//@ 
+//@ There is a crucial difference to templates in C++: We actually have to declare which traits
+//@ we want the type to satisfy. If we left away the `Minimum`, Rust would have complained that
+//@ we cannot call `min`. Just try it! <br/>
+//@ This is in strong contrast to C++, where the compiler only checks such details when the
+//@ function is actually used.
+pub fn vec_min<T: Minimum>(v: Vec<T>) -> SomethingOrNothing<T> {

     let mut min = Nothing;
     for e in v {
         min = Something(match min {
             Nothing => e,
     let mut min = Nothing;
     for e in v {
         min = Something(match min {
             Nothing => e,

-            Something(n) => T::min(n, e)
+            // Here, we can now call the `min` function of the trait.
+            Something(n) => {
+                e.min(n)                                            /*@*/
+            }

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

+//@ Before going on, take a moment to ponder the flexibility of Rust's take on abstraction:
+//@ We just defined our own, custom trait (interface), and then implemented that trait
+//@ *for an existing type*. With the hierarchical approach of, e.g., C++ or Java,
+//@ that's not possible: We cannot make an existing type suddenly also inherit from our abstract base class.
+//@ 
+//@ In case you are worried about performance, note that Rust performs *monomorphisation*
+//@ of generic functions: When you call `vec_min` with `T` being `i32`, Rust essentially goes
+//@ ahead and creates a copy of the function for this particular type, filling in all the blanks.
+//@ In this case, the call to `T::min` will become a call to our implementation *statically*. There is
+//@ no dynamic dispatch, like there would be for Java interface methods or C++ `virtual` methods.
+//@ This behavior is similar to C++ templates. The optimizer (Rust is using LLVM) then has all the
+//@ information it could want to, e.g., inline function calls.

 
 

-// To make the function usable with a `Vec<i32>`, we implement the `Minimum` trait for `i32`.
+// ## Trait implementations
+// To make `vec_min` usable with a `Vec<i32>`, we implement the `Minimum` trait for `i32`.

 impl Minimum for i32 {
 impl Minimum for i32 {

-    fn min(a: Self, b: Self) -> Self {
-        std::cmp::min(a, b)
+    fn min(self, b: Self) -> Self {
+        if self < b { self } else { b }                             /*@*/

     }
 }
 
     }
 }
 

-// In order to run our code and see the result, we again provide a `print` function.
-// This also shows that we can have multiple `impl` blocks for the same type, and we
-// can provide some methods only for certain instances of a generic type.
-impl SomethingOrNothing<i32> {
-    fn print(self) {
+// We again provide a `print` function.
+//@ This also shows that we can have multiple `impl` blocks
+//@ for the same type (remember that `NumberOrNothing` is just a type alias for `SomethingOrNothing<i32>`),
+//@ and we can provide some methods only for certain instances of a generic type.
+impl NumberOrNothing {
+    pub fn print(self) {

         match self {
             Nothing => println!("The number is: <nothing>"),
             Something(n) => println!("The number is: {}", n),
         match self {
             Nothing => println!("The number is: <nothing>"),
             Something(n) => println!("The number is: {}", n),


@@ -108,50 +129,21 @@ impl SomethingOrNothing<i32> {
     }
 }
 
     }
 }
 

-// Now we are again ready to run our code. Remember to change `main.rs` appropriately.
-// Rust figures out automatically that we want the `T` of `vec_min` to be `i32`, and
-// that `i32` implements `Minimum` and hence all is good.
-// 
-// In case you are worried about performance, note that Rust performs *monomorphisation*
-// of generic functions: When you call `vec_min` with `T` being `i32`, Rust essentially goes
-// ahead and creates a copy of the function for this particular type, filling in all the blanks.
-// In this case, the call to `T::min` will become a call to our implementation *statically*. There is
-// no dynamic dispatch, like there would be for Java interface methods or C++ `virtual` methods.
-// This behavior is similar to C++ templates. The optimizer (Rust is using LLVM) then has all the
-// information it could want to, e.g., inline function calls.
+// Now we are ready to run our new code. Remember to change `main.rs` appropriately.
+//@ Rust figures out automatically that we want the `T` of `vec_min` to be `i32`, and
+//@ that `i32` implements `Minimum` and hence all is good.

 fn read_vec() -> Vec<i32> {
     vec![18,5,7,3,9,27]
 }
 fn read_vec() -> Vec<i32> {
     vec![18,5,7,3,9,27]
 }

-pub fn part_main() {
+pub fn main() {

     let vec = read_vec();
     let min = vec_min(vec);
     min.print();
 }
 
     let vec = read_vec();
     let min = vec_min(vec);
     min.print();
 }
 

-// If this printed `3`, then you generic `vec_min` is working!
-// 
-// Before going on, take a moment to ponder the flexibility of Rust's take on abstraction:
-// We just defined our own, custom trait (interface), and then implemented that trait
-// *for an existing type*. With the hierarchical approach of, e.g., C++ or Java,
-// that's not possible: We cannot make an existing type suddenly also inherit from our abstract base class.
+//@ If this printed `3`, then you generic `vec_min` is working! So get ready for the next part.

 
 

-// **Exercise**: Define a trait `Print` to write a generic version of `SomethingOrNothing::print`.
-// Implement that trait for `i32`, and change the code above to use it.
-// I will again provide a skeleton for this solution. It also shows how to attach bounds to generic
-// implementations (just compare it to the `impl` block from the previous exercise).
-// You can read this as "For all types `T` satisfying the `Print` trait, I provide an implementation
-// for `SomethingOrNothing<T>`".
-// 
-// Notice that I called the function on `SomethingOrNothing` `print2` to disambiguate from the `print` defined above.
-// 
-// *Hint*: There is a macro `print!` for printing without appending a newline.
-trait Print {
-    /* Add things here */
-}
-impl<T: Print> SomethingOrNothing<T> {
-    fn print2(self) {
-        panic!("Not yet implemented.")
-    }
-}
+// **Exercise 02.1**: Change your program such that it computes the minimum of a `Vec<f32>` (where `f32` is the type
+// of 32-bit floating-point numbers). You should not change `vec_min` in any way, obviously!

 
 

-// [index](main.html) | [previous](part01.html) | [next](part03.html)
+//@ [index](main.html) | [previous](part01.html) | [next](part03.html)