From: Ralf Jung Date: Tue, 9 Jun 2015 09:01:11 +0000 (+0200) Subject: part 01: references, expressions, inherent impls X-Git-Url: https://git.ralfj.de/rust-101.git/commitdiff_plain/a72c9087f87e56d2fc46141e485ba66b3ca9190a?ds=sidebyside;hp=e726c740aaf22cc99aab03063df3854595bc458c part 01: references, expressions, inherent impls --- diff --git a/src/part00.rs b/src/part00.rs index 1c7d3ce..7fcbf35 100644 --- a/src/part00.rs +++ b/src/part00.rs @@ -1,5 +1,5 @@ -// 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: @@ -26,7 +26,7 @@ enum NumberOrNothing { // (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 { +fn vec_min(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": @@ -62,59 +62,13 @@ fn vec_min_try1(vec: Vec) -> NumberOrNothing { // 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 + // `vec!` is a *macro* (as you can tell from the `!`) that constructs a constant `Vec<_>` with the given // elements. } @@ -127,6 +81,9 @@ fn print_number_or_nothing(n: NumberOrNothing) { match n { Nothing => println!("The number is: "), 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. }; } diff --git a/src/part01.rs b/src/part01.rs index bd7e9fb..36e15d0 100644 --- a/src/part01.rs +++ b/src/part01.rs @@ -1,10 +1,95 @@ -// [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` denotes a *reference* to a `Vec`. 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`. +fn vec_min(v: &Vec) -> 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: "), + 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 { + 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)