copy-editing: if

I decided to break if-let out, as it's too complex for this part, but moving
if that late seems silly too.

Author: steveklabnik

src/doc/trpl/SUMMARY.md

@@ -41,6 +41,7 @@
     * [Traits](traits.md)
     * [Operators and Overloading](operators-and-overloading.md)
     * [Generics](generics.md)
+    * [if let](if-let.md)
     * [Trait Objects](trait-objects.md)
     * [Closures](closures.md)
     * [Universal Function Call Syntax](ufcs.md)

src/doc/trpl/if-let.md

@@ -0,0 +1,3 @@
+% if let
+
+COMING SOON

src/doc/trpl/if.md

@@ -1,10 +1,10 @@
 % 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.
+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
+`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.
 
@@ -20,11 +20,11 @@ if x == 5 {
 
 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.
+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`:
 
-```{rust}
+```rust
 let x = 5;
 
 if x == 5 {
@@ -50,8 +50,7 @@ if x == 5 {
 
 This is all pretty standard. However, you can also do this:
 
-
-```{rust}
+```rust
 let x = 5;
 
 let y = if x == 5 {
@@ -63,95 +62,12 @@ let y = if x == 5 {
 
 Which we can (and probably should) write like this:
 
-```{rust}
+```rust
 let x = 5;
 
 let y = if x == 5 { 10 } else { 15 }; // y: i32
 ```
 
-This reveals two interesting things about Rust: it is an expression-based
-language, and semicolons are different from semicolons 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 = 5); // expected identifier, found keyword `let`
-```
-
-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.
-
-Note that assigning to an already-bound variable (e.g. `y = 5`) is still an
-expression, although its value is not particularly useful. Unlike C, where an
-assignment evaluates to the assigned value (e.g. `5` in the previous example),
-in Rust the value of an assignment is the unit type `()` (which we'll cover later).
-
-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:
-
-```{rust}
-let x = 5;
-
-let y: i32 = if x == 5 { 10 } else { 15 };
-```
-
-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 = 5;
-
-let y: i32 = if x == 5 { 10; } else { 15; };
-```
-
-Note the semicolons after the 10 and 15. Rust will give us the following error:
-
-```text
-error: mismatched types: expected `i32`, found `()` (expected i32, found ())
-```
-
-We expected an integer, but we got `()`. `()` is pronounced *unit*, and is a
-special type in Rust's type system. In Rust, `()` is _not_ a valid value for a
-variable of type `i32`. 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.
-
-TODO: `if let`
+This works because `if` is an expression. The value of the expression is the
+value of the last expression in whichever branch was chosen. An `if` without an
+`else` always results in `()` as the value.