+// 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
+// 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.
+
+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.
+ // 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
+ // 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:
+ for el in vec {
+ // 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 {
+ NumberOrNothing::Nothing => {
+ // `min` is currently nothing, so let's just make it the number `el`.
+ min = NumberOrNothing::Number(el);
+ },
+ 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);
+ }
+ }
+ }
+ // Finally, we return the result of the computation.
+ return 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,
+ }
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