exclusive reference -> unique reference

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

diff --git a/src/part01.rs b/src/part01.rs
index 36e15d0245941e07de68de0332ec12b823b5ae37..fa06a80cda682e60ebd44c70b61546c698336411 100644 (file)
--- a/src/part01.rs
+++ b/src/part01.rs
@@ -1,27 +1,28 @@
-// Rust-101, Part 00: Expressions, Inherent methods
+// Rust-101, Part 01: Expressions, Inherent methods

 // ================================================
 
 // ================================================
 

-use std;
+// 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. 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!
+//@ 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!

 
 

-// For example, consider `sqr`:
+// ## Expression-based programming
+//@ For example, consider `sqr`:

 fn sqr(i: i32) -> i32 { i * i }
 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).
+//@ 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
+// Conditionals are also just expressions. This is comparable to the ternary `? :` operator

 // from languages like C.
 fn abs(i: i32) -> i32 { if i >= 0 { i } else { -i } }
 
 // 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.)
+//@ 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
 enum NumberOrNothing {
     Number(i32),
     Nothing


@@ -34,36 +35,45 @@ fn number_or_default(n: NumberOrNothing, default: i32) -> i32 {
     }
 }
 
     }
 }
 

-// With this fresh knowledge, let us now refactor `vec_min`. First of all, we are doing a small change
-// to the type: `&Vec<i32>` denotes a *reference* to a `Vec<i32>`. 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<i32>`.
-fn vec_min(v: &Vec<i32>) -> NumberOrNothing {
+// 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 significantly increase readability.
+    fn min_i32(a: i32, b: i32) -> i32 {
+        if a < b { a } else { b }                                   /*@*/
+    }
+

     let mut min = Nothing;
     for e in v {
     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)
-        });
+        //@ 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.
+    //@ 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.
 
     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:
+// ## 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 {
 impl NumberOrNothing {
     fn print(self) {
         match self {


@@ -72,24 +82,28 @@ impl NumberOrNothing {
         };
     }
 }
         };
     }
 }

-// 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!
+//@ 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]
 }
 
 // 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 part_main() {
+pub fn main() {

     let vec = read_vec();
     let vec = read_vec();

-    let min = vec_min(&vec);
-    min.print();
+    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.
 
 }
 // 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)
+// **Exercise 01.1**: Write a function `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)