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

diff --git a/src/part01.rs b/src/part01.rs
index 2e350bc469fef6afc760ac6a7645747276ee631a..7d29c49400e2f83f643532f812ea1b73045c2ea9 100644 (file)
--- a/src/part01.rs
+++ b/src/part01.rs
@@ -1,10 +1,109 @@
-// [index](main.html) | [previous](part00.html) | [next](part02.html)
+// Rust-101, Part 01: Expressions, Inherent methods
+// ================================================

 
 

-// Rust-101, Part 00
-// =================
+// For Rust to compile this file, make sure to enable the corresponding line
+// in `main.rs` before going on.

 
 

+//@ Even though our code from the first part works, we can still learn a
+//@ lot by making it prettier. That's because Rust is an "expression-based" language, which
+//@ 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).

 
 

-pub fn part_main() {
-    
-}
\ No newline at end of file
+// 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,
+    }
+}
+
+// It is even the case that blocks are expressions, evaluating to the last expression they contain.
+fn compute_stuff(x: i32) -> i32 {
+    let y = { let z = x*x; z + 14 };
+    y*y
+}
+
+// Let us now refactor `vec_min`.
+fn vec_min(v: Vec<i32>) -> NumberOrNothing {
+    //@ Remember that helper function `min_i32`? Rust allows us to define such helper functions *inside* other
+    //@ functions. This is just a matter of namespacing, the inner function has no access to the data of the outer
+    //@ one. Still, being able to nicely group functions can be very useful.
+    fn min_i32(a: i32, b: i32) -> i32 {
+        if a < b { a } else { b }                                   /*@*/
+    }
+
+    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) => min_i32(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.
+
+// ## 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!
+
+// 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_sum` that computes the sum of all values of a `Vec<i32>`.
+
+// **Exercise 01.2**: Write a function `vec_print` that takes a vector and prints all its elements.
+
+//@ [index](main.html) | [previous](part00.html) | [raw source](https://www.ralfj.de/git/rust-101.git/blob_plain/HEAD:/workspace/src/part01.rs) | [next](part02.html)