+//@ 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)
+//@ is called an "algebraic datatype", and Rust lets us define such types with the keyword `enum`.
+//@ 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 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":
+ 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.
+
+ // Now we want to *iterate* over the list. Rust has some nice syntax for iterators:
+ for el in vec {
+ // So `el` is an 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 = NumberOrNothing::Number(el); /*@*/
+ },
+ // In this arm, `min` is currently the number `n`, so let's compute the new minimum and
+ // store it.
+ //@ We will write the function `min_i32` just after we completed this one.
+ NumberOrNothing::Number(n) => {
+ let new_min = min_i32(n, el); /*@*/
+ min = NumberOrNothing::Number(new_min); /*@*/
+ }
+ }
+ //@ Notice that Rust makes sure you did not forget to handle any case in your `match`. We
+ //@ say that the pattern matching has to be *exhaustive*.
+ }
+ // Finally, we return the result of the computation.
+ return min;
+}
+
+// Now that we reduced the problem to computing the minimum of two integers, let's do that.
+fn min_i32(a: i32, b: i32) -> i32 {
+ if a < b {
+ return a; /*@*/
+ } else {
+ return b; /*@*/
+ }
+}
+
+// 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.
+// Try moving that above the function, and removing all the occurrences of `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.
+
+//@ `vec!` is a *macro* (as indicated by `!`) that constructs a constant `Vec<_>` with the given
+//@ elements.
+fn read_vec() -> Vec<i32> {
+ vec![18,5,7,1,9,27] /*@*/
projects / rust-101.git / blobdiff