-// Rust-101, Part 00: Algebraic datatypes, expressions
-// ===================================================
+// Rust-101, Part 00: Algebraic datatypes
+// ======================================
// As our first piece of Rust code, we want to write a function that computes the
// minimum of a list. We are going to make use of the standard library, so let's import that:
// (growable) arrays of numbers, and we will use that as our list type.
// Observe how in Rust, the return type comes *after* the arguments.
-fn vec_min_try1(vec: Vec<i32>) -> NumberOrNothing {
+fn vec_min(vec: Vec<i32>) -> NumberOrNothing {
// First, we need some variable to store the minimum as computed so far.
// Since we start out with nothing computed, this will again be a
// "number or nothing":
// the constructors of `NumberOrNothing` into the local namespace:
use self::NumberOrNothing::{Number,Nothing};
// Try moving that above the function, and removing all the occurrences `NumberOrNothing::`.
-// Things should still compile, now being much less verbose!
-
-// There is more prettification we can do. To understand how, it is important to
-// understand that Rust is an "expression-based" language. This means that most of the
-// terms you write down are not just *statements* (executing code), but *expressions*
-// (returning a value). This applies even to the body of entire functions!
-
-// For example, consider `sqr`:
-fn sqr(i: i32) -> i32 { i * i }
-// Between the curly braces, we are giving the *expression* that computes the return value.
-// So we can just write `i * i`, the expression that returns the square if `i`!
-// This is very close to how mathematicians write down functions (but with more types).
-
-// Conditionals are also just expressions. You can compare this to the ternary `? :` operator
-// from languages like C.
-fn abs(i: i32) -> i32 { if i >= 0 { i } else { -i } }
-
-// And the same applies to case distinction with `match`: Every `arm` of the match
-// gives the expression that is returned in the respective case.
-fn number_or_default(n: NumberOrNothing, default: i32) -> i32 {
- match n {
- Nothing => default,
- Number(n) => n,
- }
-}
-
-// With this fresh knowledge, let us now refactor `vec_min`.
-fn vec_min(v: Vec<i32>) -> NumberOrNothing {
- let mut min = Nothing;
- for e in v {
- // First of all, notice that all we do here is compute a new value for `min`, and that it
- // will always end up being `Number` rather than `Nothing`. In Rust, the structure of the code
- // can express this uniformity as follows:
- min = Number(match min {
- Nothing => e,
- Number(n) => std::cmp::min(n, e)
- });
- }
- // The `return` keyword exists in Rust, but it is rarely used. Instead, we typically
- // make use of the fact that the entire function body is an expression, so we can just
- // write down the desired return value.
- min
-}
-
-// Now that's already much shorter! Make sure you can go over the code above and actually understand
-// every step of what's going on.
// To call this function, we now just need a list. Of course, ultimately we want to ask the user for
// a list of numbers, but for now, let's just hard-code something:
fn read_vec() -> Vec<i32> {
vec![18,5,7,1,9,27]
- // `vec!` is a *macro* (as you can tell from the `!`) that constructs a constant `Vec` with the given
+ // `vec!` is a *macro* (as you can tell from the `!`) that constructs a constant `Vec<_>` with the given
// elements.
}
match n {
Nothing => println!("The number is: <nothing>"),
Number(n) => println!("The number is: {}", n),
+ // `println!` is again a macro, where the first argument is a *format string*. For
+ // now, you just need to know that `{}` is the placeholder for a value, and that Rust
+ // will check at compile-time that you supplied the right number of arguments.
};
}
-// [index](main.html) | [previous](part00.html) | [next](part02.html)
+// Rust-101, Part 00: Expressions, Inherent methods
+// ================================================
+
+use std;
+
+// Even though our code from the first part works, we can still learn a
+// lot by making it prettier. To understand how, it is important to
+// understand that Rust is an "expression-based" language. This means that most of the
+// terms you write down are not just *statements* (executing code), but *expressions*
+// (returning a value). This applies even to the body of entire functions!
+
+// For example, consider `sqr`:
+fn sqr(i: i32) -> i32 { i * i }
+// Between the curly braces, we are giving the *expression* that computes the return value.
+// So we can just write `i * i`, the expression that returns the square if `i`!
+// This is very close to how mathematicians write down functions (but with more types).
-// Rust-101, Part 00
-// =================
+// Conditionals are also just expressions. You can compare this to the ternary `? :` operator
+// from languages like C.
+fn abs(i: i32) -> i32 { if i >= 0 { i } else { -i } }
+
+// And the same applies to case distinction with `match`: Every `arm` of the match
+// gives the expression that is returned in the respective case.
+// (We repeat the definition from the previous part here.)
+enum NumberOrNothing {
+ Number(i32),
+ Nothing
+}
+use self::NumberOrNothing::{Number,Nothing};
+fn number_or_default(n: NumberOrNothing, default: i32) -> i32 {
+ match n {
+ Nothing => default,
+ Number(n) => n,
+ }
+}
+// With this fresh knowledge, let us now refactor `vec_min`. First of all, we are doing a small change
+// to the type: `&Vec<i32>` denotes a *reference* to a `Vec<i32>`. You can think of this as a pointer
+// (in C terms): Arguments in Rust are passed *by value*, so we need to employ explicit references if
+// that's not what we want. References are per default immutable (like variables), a mutable reference
+// would be denoted `&mut Vec<i32>`.
+fn vec_min(v: &Vec<i32>) -> NumberOrNothing {
+ let mut min = Nothing;
+ for e in v {
+ let e = *e;
+ // Notice that all we do here is compute a new value for `min`, and that it will always end
+ // up being a `Number` rather than `Nothing`. In Rust, the structure of the code
+ // can express this uniformity as follows:
+ min = Number(match min {
+ Nothing => e,
+ Number(n) => std::cmp::min(n, e)
+ });
+ }
+ // The `return` keyword exists in Rust, but it is rarely used. Instead, we typically
+ // make use of the fact that the entire function body is an expression, so we can just
+ // write down the desired return value.
+ min
+}
+
+// Now that's already much shorter! Make sure you can go over the code above and actually understand
+// every step of what's going on.
+
+// So much for `vec_min`. Let us now reconsider `print_number_or_nothing`. That function
+// really belongs pretty close to the type `NumberOrNothing`. In C++ or Java, you would
+// probably make it a method of the type. In Rust, we can achieve something very similar
+// by providing an *inherent implementation* as follows:
+impl NumberOrNothing {
+ fn print(self) {
+ match self {
+ Nothing => println!("The number is: <nothing>"),
+ Number(n) => println!("The number is: {}", n),
+ };
+ }
+}
+// So, what just happened? Rust separates code from data, so the definition of the
+// methods on an `enum` (and also on `struct`, which we will learn about later)
+// is independent of the definition of the type. `self` is like `this` in other
+// languages, and its type is always implicit. So `print` is now a method that
+// takes as first argument a `NumberOrNothing`, just like `print_number_or_nothing`.
+//
+// Try making `number_or_default` from above an inherent method as well!
+
+// With our refactored functions and methods, `main` now looks as follows:
+fn read_vec() -> Vec<i32> {
+ vec![18,5,7,2,9,27]
+}
pub fn part_main() {
-
+ let vec = read_vec();
+ let min = vec_min(&vec);
+ min.print();
}
+// You will have to replace `part00` by `part01` in the `main` function in
+// `main.rs` to run this code.
// [index](main.html) | [previous](part00.html) | [next](part02.html)
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