X-Git-Url: https://git.ralfj.de/rust-101.git/blobdiff_plain/4c7f5b23c9bd4cedc1374fd692edc5edf06b701f..2707b4455698e461271b2a4253753b6cb5503751:/src/part00.rs?ds=sidebyside diff --git a/src/part00.rs b/src/part00.rs index 962e801..1c7d3ce 100644 --- a/src/part00.rs +++ b/src/part00.rs @@ -1,21 +1,147 @@ -// [index](main.html) | previous | [next](part01.html) +// Rust-101, Part 00: Algebraic datatypes, expressions +// =================================================== + +// 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: -// Rust-101, Part 00 -// ================= +use std; -// As a starter, we want to write a function that computes the minimum of a list. -// First, we need to get a list. For now, let's just hard-code some sample list. -// Later, we will ask the user to input a list. -// -// Observe how in Rust, the function type comes *after* the arguments (which, in -// this case, the function does not have). `Vec` is the type of a growable list -// of (signed, 32-bit) integers. +// Let us start by thinking about the *type* of our function. Rust forces us to give the types of +// all arguments, and the return type, before we even start writing the body. In the case of our minimum +// function, we may be inclined to say that it returns a number. But then we would be in trouble: What's +// the minimum of an empty list? The type of the function says we have to return *something*. +// We could just choose 0, but that would be kind of arbitrary. What we need +// is a type that is "a number, or nothing". Such a type (of multiple exclusive options) +// is called an "algebraic datatype", and Rust lets us define such types with the keyword `enum`. +// 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. -pub fn read_vec() -> Vec { - vec![0,1,2,3,4] +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. + // Since we start out with nothing computed, this will again be a + // "number or nothing": + let mut min = NumberOrNothing::Nothing; + // We do not have to write a type next to `min`, Rust can figure that out automatically + // (a bit like `auto` in C++11). Also notice the `mut`: In Rust, variables are + // immutable per default, and you need to tell Rust if you want + // to change a variable later. + + // Now we want to *iterate* over the list. Rust has some nice syntax for + // iterators: + for el in vec { + // 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 { + 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); + }, + 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); + } + } + } + // Finally, we return the result of the computation. + return min; +} + +// 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 +} + +// 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 { + 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! +// But, wait, we would like to actually see something, so we need to print the result. +// 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. + +fn print_number_or_nothing(n: NumberOrNothing) { + match n { + Nothing => println!("The number is: "), + Number(n) => println!("The number is: {}", n), + }; +} + +// Putting it all together: pub fn part_main() { - + let vec = read_vec(); + let min = vec_min(vec); + print_number_or_nothing(min); } + +// Now try `cargo run` on the console to run above code. + +// Yay, it said "1"! That's actually the right answer. Okay, we could have +// computed that ourselves, but that's besides the point. More importantly: +// You completed the first part of the course. + +// [index](main.html) | previous | [next](part01.html)