-// [index](main.html) | previous | [next](part01.html)
+// Rust-101, Part 00: Algebraic datatypes
+// ======================================
-use std;
+// As our first piece of Rust code, we want to write a function that computes the
+// minimum of a list.
-// Rust-101, Part 00: Algebraic datatypes, expressions
-// ===================================================
+// We are going to make use of the standard library, so let's import that:
+use std;
-// As a starter, we want to write a function that computes the minimum of a list.
-// First, we need to write down the signature of the function: The types of its arguments and
-// of the return value. 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
+// 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)
// 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<i32>` is the type of
// (growable) arrays of numbers, and we will use that as our list type.
-// Observe how in Rust, the function type comes *after* the arguments.
-fn vec_min_try1(vec: Vec<i32>) -> NumberOrNothing {
- // First, we need some variable to store the minimum as computed so far.
+// Observe how in Rust, the return type comes *after* the arguments.
+fn vec_min(vec: Vec<i32>) -> 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". Notice that 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
+ // "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.
- let mut min = NumberOrNothing::Nothing;
// Now we want to *iterate* over the list. Rust has some nice syntax for
// iterators:
// So `el` is al 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` is currently nothing, so let's just make it the number `el`.
min = NumberOrNothing::Number(el);
},
+ // In this arm, `min` is currently the number `n`, so let's compute the new minimum and store it.
NumberOrNothing::Number(n) => {
- // `min` is currently the number `n`, so let's compute the new minimum and store it.
let new_min = std::cmp::min(n, el);
min = NumberOrNothing::Number(new_min);
}
}
// 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:
-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!
-
-// However, the code is still not "idiomatic Rust code". To understand why, 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`. 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`, and make that our return value! Note that this is
-// very close to how mathematicians write down functions (but with more types).
-fn sqr(i: i32) -> i32 { i * i }
-
-// 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<i32>) -> 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 we
- // will always end up calling the `Number` constructor. 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
-}
+// ugly. Can't we do that nicer?
-// 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.
+// 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 `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:
+// 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 you can tell from the `!`) that constructs a constant `Vec<_>` with the given
+// elements.
fn read_vec() -> Vec<i32> {
- // `vec!` is a *macro* (as you can tell from the `!`) that constructs a constant `Vec` with the given
- // elements.
vec![18,5,7,1,9,27]
}
// Finally, let's call our functions and run the code!
-// But, wait, we would like to actually see something. Of course Rust can print numbers,
-// but after calling `vec_min`, we have a `NumberOrNothing`. So let's write a small helper
-// function that can prints such values.
+// But, wait, we would like to actually see something, 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: <nothing>"),
};
}
-// So putting it all together - if you type `cargo run`, it will
-// run the following code:
-
+// Putting it all together:
pub fn part_main() {
let vec = read_vec();
let min = vec_min(vec);
print_number_or_nothing(min);
}
+// Now try `cargo run` on the console to run above code.
+
// Yay, it said "1"! That's actually the right answer. Okay, we could have
// computed that ourselves, but that's besides the point. More importantly:
// You completed the first part of the course.
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