X-Git-Url: https://git.ralfj.de/rust-101.git/blobdiff_plain/800c78a128bd22bbe700164de5019ff8270fd746..46c141eefadadaf82b1414ae19d3766bbd4ba0cc:/src/part00.rs diff --git a/src/part00.rs b/src/part00.rs index 308c242..4deb829 100644 --- a/src/part00.rs +++ b/src/part00.rs @@ -1,14 +1,16 @@ -// [index](main.html) | previous | [next](part01.html) +// Rust-101, Part 00: Algebraic datatypes +// ====================================== -use std; +// As our first piece of Rust code, we want to write a function that computes the +// minimum of a list. -// Rust-101, Part 00: Algebraic datatypes, expressions -// =================================================== +// We are going to make use of the standard library, so let's import that: +use std; -// As a starter, we want to write a function that computes the minimum of a list. -// First, we need to write down the signature of the function: The types of its arguments and -// of the return value. 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 +// ## Getting started +// 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) @@ -16,25 +18,25 @@ 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 function 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". Notice that 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 + // "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. - let mut min = NumberOrNothing::Nothing; // Now we want to *iterate* over the list. Rust has some nice syntax for // iterators: @@ -42,12 +44,12 @@ 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 => { - // `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) => { - // `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); } @@ -58,71 +60,30 @@ 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! - -// However, the code is still not "idiomatic Rust code". To understand why, 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`. 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`, and make that our return value! Note that this is -// very close to how mathematicians write down functions (but with more types). -fn sqr(i: i32) -> i32 { i * i } - -// 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 we - // will always end up calling the `Number` constructor. 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: +// 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. +// `vec!` is a *macro* (as you can tell from the `!`) that constructs a constant `Vec<_>` with the given +// elements. fn read_vec() -> Vec { - // `vec!` is a *macro* (as you can tell from the `!`) that constructs a constant `Vec` with the given - // elements. vec![18,5,7,1,9,27] } // Finally, let's call our functions and run the code! -// But, wait, we would like to actually see something. Of course Rust can print numbers, -// but after calling `vec_min`, we have a `NumberOrNothing`. So let's write a small helper -// function that can prints such values. +// 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. +// `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: "), @@ -130,15 +91,15 @@ fn print_number_or_nothing(n: NumberOrNothing) { }; } -// So putting it all together - if you type `cargo run`, it will -// run the following code: - -pub fn part_main() { +// Putting it all together: +pub fn 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.