X-Git-Url: https://git.ralfj.de/rust-101.git/blobdiff_plain/e48d2870f91b769680b0cd12895066a94c4131aa..fb6cc06498be64fa09f04b5ed847f1752e0eba49:/src/part00.rs diff --git a/src/part00.rs b/src/part00.rs index 585df9b..00eb4d1 100644 --- a/src/part00.rs +++ b/src/part00.rs @@ -1,10 +1,116 @@ -// [index](main.html) | previous | [next](part01.html) +// Rust-101, Part 00: Algebraic datatypes +// ====================================== -// Rust-101, Part 00 -// ================= +// As our first piece of Rust code, we want to write a function that computes the +// minimum of a list. +//@ ## 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) +//@ 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. +// 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. -pub fn part_main() { - -} \ No newline at end of file +// 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; + //@ 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 an 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 = NumberOrNothing::Number(el); /*@*/ + }, + // In this arm, `min` is currently the number `n`, so let's compute the new minimum and store it. + //@ We will write the function `min_i32` just after we completed this one. + NumberOrNothing::Number(n) => { + let new_min = min_i32(n, el); /*@*/ + min = NumberOrNothing::Number(new_min); /*@*/ + } + } + //@ Notice that Rust makes sure you did not forget to handle any case in your `match`. We say + //@ that the pattern matching has to be *exhaustive*. + } + // Finally, we return the result of the computation. + return min; +} + +// Now that we reduced the problem to computing the minimum of two integers, let's do that. +fn min_i32(a: i32, b: i32) -> i32 { + if a < b { + return a; /*@*/ + } else { + return b; /*@*/ + } +} + +// 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. +// Try moving that above the function, and removing all the occurrences of `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. + +//@ `vec!` is a *macro* (as indicated by `!`) that constructs a constant `Vec<_>` with the given +//@ elements. +fn read_vec() -> Vec { + vec![18,5,7,1,9,27] /*@*/ +} + +// Of course, we would also like to actually see the result of the computation, 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: "), /*@*/ + Number(n) => println!("The number is: {}", n), /*@*/ + }; /*@*/ +} + +// Putting it all together: +pub fn main() { + let vec = read_vec(); + let min = vec_min(vec); + print_number_or_nothing(min); +} + +//@ You can now use `cargo build` to compile your *crate*. That's Rust's name for a *compilation unit*, which in +//@ the case of Rust means an application or a library.
+// Finally, try `cargo run` on the console to run it. + +//@ Yay, it said "1"! That's actually the right answer. Okay, we could have +//@ computed that ourselves, but that's beside the point. More importantly: +//@ You completed the first part of the course. + +//@ [index](main.html) | previous | [raw source](https://www.ralfj.de/git/rust-101.git/blob_plain/HEAD:/workspace/src/part00.rs) | [next](part01.html)