-// Rust-101, Part 02: Generic types (WIP)
-// ================================
+// Rust-101, Part 02: Generic types, Traits
+// ========================================
use std;
// 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 `SomethingOrNothing` type.
+// a generic type `SomethingOrNothing`.
enum SomethingOrNothing<T> {
Something(T),
Nothing,
// **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).
//
// *using* that variable ("The thing I do, is implement `SomethingOrNothing<T>`").
impl<T> SomethingOrNothing<T> {
fn new(o: Option<T>) -> Self {
- panic!("Not yet implemented.");
+ panic!("Not yet implemented.")
}
fn to_option(self) -> Option<T> {
- panic!("Not yet implemented.");
+ panic!("Not yet implemented.")
}
}
// Inside an `impl`, `Self` refers to the type we are implementing things for. Here, it is
fn call_constructor(x: i32) -> SomethingOrNothing<i32> {
SomethingOrNothing::new(Some(x))
}
-
-// [index](main.html) | [previous](part01.html) | next
+// 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;
+}
+
+// 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> {
+ let mut min = Nothing;
+ for e in v {
+ let e = *e;
+ min = Something(match min {
+ Nothing => e,
+ Something(n) => T::min(n, e)
+ });
+ }
+ min
+}
+
+// To make the function usable with a `Vec<i32>`, we implement the `Minimum` trait for `i32`.
+impl Minimum for i32 {
+ fn min(a: Self, b: Self) -> Self {
+ std::cmp::min(a, 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) {
+ match self {
+ Nothing => println!("The number is: <nothing>"),
+ Something(n) => println!("The number is: {}", n),
+ };
+ }
+}
+
+// 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.
+fn read_vec() -> Vec<i32> {
+ vec![18,5,7,3,9,27]
+}
+pub fn part_main() {
+ 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.
+
+// **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.")
+ }
+}
+
+// [index](main.html) | [previous](part01.html) | [next](part03.html)
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