X-Git-Url: https://git.ralfj.de/rust-101.git/blobdiff_plain/bba83d98eea727e675db05386b9c3f1dc3b51c47..41c94f98f6cf8b5a05a8c3eb8d13bb73fc4fda3f:/src/part00.rs diff --git a/src/part00.rs b/src/part00.rs index 1c7d3ce..90b04ef 100644 --- a/src/part00.rs +++ b/src/part00.rs @@ -1,9 +1,10 @@ -// 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: +// minimum of a list. +// We are going to make use of the standard library, so let's import that: use std; // Let us start by thinking about the *type* of our function. Rust forces us to give the types of @@ -16,18 +17,18 @@ use std; // Coming from C(++), you can think of such a type as a `union`, together with a field that // stores the variant of the union that's currently used. +// An `enum` for "a number or nothing" could look as follows: enum NumberOrNothing { Number(i32), Nothing } - // Notice that `i32` is the type of (signed, 32-bit) integers. To write down the type of // the minimum function, we need just one more ingredient: `Vec` is the type of // (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) -> NumberOrNothing { - // First, we need some variable to store the minimum as computed so far. +// Observe how in Rust, the return type comes *after* the arguments. +fn vec_min(vec: Vec) -> NumberOrNothing { + // In the function, we first 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": let mut min = NumberOrNothing::Nothing; @@ -42,12 +43,12 @@ fn vec_min_try1(vec: Vec) -> NumberOrNothing { // So `el` is al element of the list. We need to update `min` accordingly, but how do we get the current // number in there? This is what pattern matching can do: match min { + // In this case (*arm*) of the `match`, `min` is currently nothing, so let's just make it the number `el`. NumberOrNothing::Nothing => { - // In this case (*arm*) of the `match`, `min` is currently nothing, so let's just make it the number `el`. min = NumberOrNothing::Number(el); }, + // In this arm, `min` is currently the number `n`, so let's compute the new minimum and store it. NumberOrNothing::Number(n) => { - // In this arm, `min` is currently the number `n`, so let's compute the new minimum and store it. let new_min = std::cmp::min(n, el); min = NumberOrNothing::Number(new_min); } @@ -58,64 +59,20 @@ fn vec_min_try1(vec: Vec) -> NumberOrNothing { } // Phew. We wrote our first Rust function! But all this `NumberOrNothing::` is getting kind of -// ugly. Can't we do that nicer? Indeed, we can: The following line tells Rust to take -// 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) -> 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 -} +// ugly. Can't we do that nicer? -// 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. +// Indeed, we can: The following line tells Rust to take +// the constructors of `NumberOrNothing` into the local namespace. +// Try moving that above the function, and removing all the occurrences `NumberOrNothing::`. +use self::NumberOrNothing::{Number,Nothing}; // 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: +// a list of numbers, but for now, let's just hard-code something. +// `vec!` is a *macro* (as you can tell from the `!`) that constructs a constant `Vec<_>` with the given +// elements. fn read_vec() -> Vec { 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 - // elements. } // Finally, let's call our functions and run the code! @@ -123,6 +80,9 @@ fn read_vec() -> Vec { // Of course Rust can print numbers, but after calling `vec_min`, we have a `NumberOrNothing`. // So let's write a small helper function that prints such values. +// `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. fn print_number_or_nothing(n: NumberOrNothing) { match n { Nothing => println!("The number is: "), @@ -131,7 +91,6 @@ fn print_number_or_nothing(n: NumberOrNothing) { } // Putting it all together: - pub fn part_main() { let vec = read_vec(); let min = vec_min(vec);