Guide: if

Author: steveklabnik

src/doc/guide.md

@@ -611,6 +611,153 @@ concept: `if`.
 
 ## If
 
+Rust's take on `if` is not particularly complex, but it's much more like the
+`if` you'll find in a dynamically typed language than in a more traditional
+systems language. So let's talk about it, to make sure you grasp the nuances.
+
+`if` is a specific form of a more general concept, the 'branch.' The name comes
+from a branch in a tree: a decision point, where depending on a choice,
+multiple paths can be taken.
+
+In the case of `if`, there is one choice that leads down two paths:
+
+```rust
+let x = 5i;
+
+if x == 5i {
+    println!("x is five!");
+}
+```
+
+If we changed the value of `x` to something else, this line would not print.
+More specifically, if the expression after the `if` evaluates to `true`, then
+the block is executed. If it's `false`, then it is not.
+
+If you want something to happen in the `false` case, use an `else`:
+
+```
+let x = 5i;
+
+if x == 5i {
+    println!("x is five!");
+} else {
+    println!("x is not five :(");
+}
+```
+
+This is all pretty standard. However, you can also do this:
+
+
+```
+let x = 5i;
+
+let y = if x == 5i {
+    10i
+} else {
+    15i
+};
+```
+
+Which we can (and probably should) write like this:
+
+```
+let x = 5i;
+
+let y = if x == 5i { 10i } else { 15i };
+```
+
+This reveals two interesting things about Rust: it is an expression-based
+language, and semicolons are different than in other 'curly brace and
+semicolon'-based languages. These two things are related.
+
+### Expressions vs. Statements
+
+Rust is primarily an expression based language. There are only two kinds of
+statements, and everything else is an expression.
+
+So what's the difference? Expressions return a value, and statements do not.
+In many languages, `if` is a statement, and therefore, `let x = if ...` would
+make no sense. But in Rust, `if` is an expression, which means that it returns
+a value. We can then use this value to initialize the binding.
+
+Speaking of which, bindings are a kind of the first of Rust's two statements.
+The proper name is a **declaration statement**. So far, `let` is the only kind
+of declaration statement we've seen. Let's talk about that some more.
+
+In some languages, variable bindings can be written as expressions, not just
+statements. Like Ruby:
+
+```{ruby}
+x = y = 5
+```
+
+In Rust, however, using `let` to introduce a binding is _not_ an expression. The
+following will produce a compile-time error:
+
+```{ignore}
+let x = (let y = 5i); // found `let` in ident position
+```
+
+The compiler is telling us here that it was expecting to see the beginning of
+an expression, and a `let` can only begin a statement, not an expression.
+
+However, re-assigning to a mutable binding is an expression:
+
+```{rust}
+let mut x = 0i;
+let y = x = 5i;
+```
+
+In this case, we have an assignment expression (`x = 5`) whose value is
+being used as part of a `let` declaration statement (`let y = ...`).
+
+The second kind of statement in Rust is the **expression statement**. Its
+purpose is to turn any expression into a statement. In practical terms, Rust's
+grammar expects statements to follow other statements. This means that you use
+semicolons to separate expressions from each other. This means that Rust
+looks a lot like most other languages that require you to use semicolons
+at the end of every line, and you will see semicolons at the end of almost
+every line of Rust code you see.
+
+What is this exception that makes us say 'almost?' You saw it already, in this
+code:
+
+```
+let x = 5i;
+
+let y: int = if x == 5i { 10i } else { 15i };
+```
+
+Note that I've added the type annotation to `y`, to specify explicitly that I
+want `y` to be an integer.
+
+This is not the same as this, which won't compile:
+
+```{ignore}
+let x = 5i;
+
+let y: int = if x == 5 { 10i; } else { 15i; };
+```
+
+Note the semicolons after the 10 and 15. Rust will give us the following error:
+
+```{ignore,notrust}
+error: mismatched types: expected `int` but found `()` (expected int but found ())
+```
+
+We expected an integer, but we got `()`. `()` is pronounced 'unit', and is a
+special type in Rust's type system. `()` is different than `null` in other
+languages, because `()` is distinct from other types. For example, in C, `null`
+is a valid value for a variable of type `int`. In Rust, `()` is _not_ a valid
+value for a variable of type `int`. It's only a valid value for variables of
+the type `()`, which aren't very useful. Remember how we said statements don't
+return a value? Well, that's the purpose of unit in this case. The semicolon
+turns any expression into a statement by throwing away its value and returning
+unit instead.
+
+There's one more time in which you won't see a semicolon at the end of a line
+of Rust code. For that, we'll need our next concept: functions.
+
 ## Functions
 
 return