Guide: functions

Just a few words about functions and defining them.

src/doc/guide.md

@@ -760,6 +760,152 @@ of Rust code. For that, we'll need our next concept: functions.
 
 ## Functions
 
+You've already seen one function so far, the `main` function:
+
+```{rust}
+fn main() {
+}
+```
+
+This is the simplest possible function declaration. As we mentioned before,
+`fn` says 'this is a function,' followed by the name, some parenthesis because
+this function takes no arguments, and then some curly braces to indicate the
+body. Here's a function named `foo`:
+
+```{rust}
+fn foo() {
+}
+```
+
+So, what about taking arguments? Here's a function that prints a number:
+
+```{rust}
+fn print_number(x: int) {
+    println!("x is: {}", x);
+}
+```
+
+Here's a complete program that uses `print_number`:
+
+```{rust}
+fn main() {
+    print_number(5);
+}
+
+fn print_number(x: int) {
+    println!("x is: {}", x);
+}
+```
+
+As you can see, function arguments work very similar to `let` declarations:
+you add a type to the argument name, after a colon.
+
+Here's a complete program that adds two numbers together and prints them:
+
+```{rust}
+fn main() {
+    print_sum(5, 6);
+}
+
+fn print_sum(x: int, y: int) {
+    println!("sum is: {}", x + y);
+}
+```
+
+You separate arguments with a comma, both when you call the function, as well
+as when you declare it.
+
+Unlike `let`, you _must_ declare the types of function arguments. This does
+not work:
+
+```{ignore}
+fn print_number(x, y) {
+    println!("x is: {}", x + y);
+}
+```
+
+You get this error:
+
+```{ignore,notrust}
+hello.rs:5:18: 5:19 error: expected `:` but found `,`
+hello.rs:5 fn print_number(x, y) {
+```
+
+This is a deliberate design decision. While full-program inference is possible,
+languages which have it, like Haskell, often suggest that documenting your
+types explicitly is a best-practice. We agree that forcing functions to declare
+types while allowing for inference inside of function bodies is a wonderful
+compromise between full inference and no inference.
+
+What about returning a value? Here's a function that adds one to an integer:
+
+```{rust}
+fn add_one(x: int) -> int {
+    x + 1
+}
+```
+
+Rust functions return exactly one value, and you declare the type after an
+'arrow', which is a dash (`-`) followed by a greater-than sign (`>`).
+
+You'll note the lack of a semicolon here. If we added it in:
+
+```{ignore}
+fn add_one(x: int) -> int {
+    x + 1;
+}
+```
+
+We would get an error:
+
+```{ignore,notrust}
+note: consider removing this semicolon:
+     x + 1;
+          ^
+error: not all control paths return a value
+fn add_one(x: int) -> int {
+     x + 1;
+}
+```
+
+Remember our earlier discussions about semicolons and `()`? Our function claims
+to return an `int`, but with a semicolon, it would return `()` instead. Rust
+realizes this probably isn't what we want, and suggests removing the semicolon.
+
+This is very much like our `if` statement before: the result of the block
+(`{}`) is the value of the expression. Other expression-oriented languages,
+such as Ruby, work like this, but it's a bit unusual in the systems programming
+world. When people first learn about this, they usually assume that it
+introduces bugs. But because Rust's type system is so strong, and because unit
+is its own unique type, we have never seen an issue where adding or removing a
+semicolon in a return position would cause a bug.
+
+But what about early returns? Rust does have a keyword for that, `return`:
+
+```{rust}
+fn foo(x: int) -> int {
+    if x < 5 { return x; }
+
+    x + 1
+}
+```
+
+Using a `return` as the last line of a function works, but is considered poor
+style:
+
+```{rust}
+fn foo(x: int) -> int {
+    if x < 5 { return x; }
+
+    return x + 1;
+}
+```
+
+There are some additional ways to define functions, but they involve features
+that we haven't learned about yet, so let's just leave it at that for now.
+
+## Comments
+
 return
 
 comments