tune and tweak: part 05-07

[rust-101.git] / src / part01.rs

diff --git a/src/part01.rs b/src/part01.rs
index c4c21c5ef0eb6d493f9376a5743a5541321eb723..dc97d8dbc1d957e29c5009207809523c0ea3d793 100644 (file)
--- a/src/part01.rs
+++ b/src/part01.rs
@@ -1,12 +1,96 @@
-// [index](main.html) | [previous](part00.html) | [next](part02.html)
+// Rust-101, Part 01: 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!
+
+// ## Expression-based programming
+// 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.
+// (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`.
+fn vec_min(v: Vec<i32>) -> NumberOrNothing {
+    let mut min = Nothing;
+    for e in v {
+        // 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.
+        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
+}

 
 

-// Rust-101, Part 00
-// =================
+// 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.

 
 

-pub fn read_vec() -> Vec<i32> {
-    vec![0,1,2,3,4]
+// ## Inherent implementations
+// 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*.
+impl NumberOrNothing {
+    fn print(self) {
+        match self {
+            Nothing => println!("The number is: <nothing>"),
+            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!

 
 

-pub fn part_main() {
-    
+// With our refactored functions and methods, `main` now looks as follows:
+fn read_vec() -> Vec<i32> {
+    vec![18,5,7,2,9,27]
+}
+pub fn 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.
+
+// **Exercise 01.1**: Write a funtion `vec_avg` that computes the average value of a `Vec<i32>`.
+// 
+// *Hint*: `vec.len()` returns the length of a vector `vec`.
+
+// [index](main.html) | [previous](part00.html) | [next](part02.html)