// 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> {
+pub enum SomethingOrNothing<T> {
Something(T),
Nothing,
}
-use self::SomethingOrNothing::{Something,Nothing};
+// 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`, 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)!
+// `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).
+// ## 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.
+// **Exercise 02.1**: Implement such functions! I provided a skeleton of the solution. Here,
+// `unimplemented!` is another macro. This one terminates execution saying that something has not yet
+// been implemented.
//
// 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 {
- panic!("Not yet implemented.")
+ unimplemented!()
}
fn to_option(self) -> Option<T> {
- panic!("Not yet implemented.")
+ unimplemented!()
}
}
-// 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:
+//
+// 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))
}
+// ## 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;
+// 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`.
+// 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.
//
-// Notice a crucial difference to templates in C++: We actually have to declare which traits
+// 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! There is no reason to believe that `T` provides such an operation.
+// 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.
-fn vec_min<T: Minimum>(v: Vec<T>) -> SomethingOrNothing<T> {
+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,
- Something(n) => T::min(n, e)
+ Something(n) => e.min(n)
});
}
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.
+// ## Trait implementations
// 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)
+ 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),
// 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]
}
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 02.2**: 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!
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