Write the 'primitive types' section of TRPL

A brief introduction to each type, with pointers to the primitive pages
for more info.

Author: steveklabnik

src/doc/trpl/SUMMARY.md

@@ -17,8 +17,8 @@
     * [`Deref` coercions](deref-coercions.md)
 * [Syntax and Semantics](syntax-and-semantics.md)
     * [Variable Bindings](variable-bindings.md)
-    * [Primitive Types](primitive-types.md)
     * [Functions](functions.md)
+    * [Primitive Types](primitive-types.md)
     * [Comments](comments.md)
     * [Structs](structs.md)
     * [Mutability](mutability.md)
@@ -35,7 +35,6 @@
     * [Move semantics](move-semantics.md)
     * [Drop](drop.md)
     * [Vectors](vectors.md)
-    * [Arrays](arrays.md)
     * [Slices](slices.md)
     * [Strings](strings.md)
     * [Traits](traits.md)

src/doc/trpl/arrays.md

@@ -1,3 +1,283 @@
 % Primitive Types
 
-Coming Soon!
+The Rust language has a number of types that are considered ‘primitive’. This
+means that they’re built-in to the language. Rust is structured in such a way
+that the standard library also provides a number of useful types built on top
+of these ones, as well, but these are the most primitive.
+
+
+# Unit, or `()`
+
+The most basic built-in type is `()`, pronounced ‘unit’. The unit type has a
+single valid value, which is also written `()`.
+
+The unit type is used as the default return type of functions:
+
+```rust
+fn foo() {
+# }
+```
+
+and
+
+```rust
+fn foo() -> () {
+# }
+```
+
+Are equivalent. This is the most common case for `()`: the value returned when
+you don’t want to return anything. Another example is the value of `let`:
+
+```rust
+let x = 5;
+```
+
+The value of this line is `()`.
+
+# Booleans
+
+Rust has a built in boolean type, named `bool`. It has two values, `true` and `false`:
+
+```rust
+let x = true;
+
+let y: bool = false;
+```
+
+A common use of booleans is in [`if` statements][if].
+
+[if]: if.html
+
+You can find more documentation for `bool`s [in the standard library
+documentation][bool].
+
+[bool]: ../std/primitive.bool.html
+
+# `char`
+
+The `char` type represents a single Unicode scalar value. You can create `char`s
+with a single tick: (`'`)
+
+```rust
+let x = 'x';
+let two_hearts = '💕';
+```
+
+Unlike some other languages, this means that Rust’s `char` is not a single byte,
+but four.
+
+You can find more documentation for `char`s [in the standard library
+documentation][char].
+
+[char]: ../std/primitive.char.html
+
+# Numeric types
+
+Rust has a variety of numeric types in a few categories: signed and unsigned,
+fixed and variable, floating-point and integer.
+
+These types consist of two parts: the category, and the size. For example,
+`u16` is an unsigned type with sixteen bits of size. More bits lets you have
+bigger numbers.
+
+If a number literal has nothing to cause its type to be inferred, it defaults:
+
+```rust
+let x = 42; // x has type i32
+
+let y = 1.0; // y has type f64
+```
+
+## Signed and Unsigned
+
+Integer types come in two varieties: signed and unsigned. To understand the
+difference, let’s consider a number with four bits of size. A signed, four-bit
+number would let you store numbers from `-8` to `+7`. Signed numbers use
+‘two’s compliment representation’. An unsigned four bit number, since it does
+not need to store negatives, can store values from `0` to `+15`.
+
+Unsigned types use a `u` for their category, and signed types use `i`. The `i`
+is for ‘integer’. So `u8` is an eight-bit unsigned number, and `i8` is an
+eight-bit signed number. 
+
+## Fixed size types
+
+Fixed size types have a specific number of bits in their representation. Valid
+bit sizes are `8`, `16`, `32`, and `64`. So, `u32` is an unsigned, 32-bit integer,
+and `i64` is a signed, 64-bit integer.
+
+## Variable sized types
+
+Rust also provides types whose size depends on the size of a pointer of the
+underlying machine. These types have ‘size’ as the category, and come in signed
+and unsigned varieties. This makes for two types: `isize` and `usize`.
+
+## Floating-point types
+
+Rust also two floating point types: `f32` and `f64`. These correspond to 
+IEEE-754 single and double precision numbers.
+
+# Arrays
+
+Like many programming languages, Rust has list types to represent a sequence of
+things. The most basic is the *array*, a fixed-size list of elements of the
+same type. By default, arrays are immutable.
+
+```rust
+let a = [1, 2, 3]; // a: [i32; 3]
+let mut m = [1, 2, 3]; // mut m: [i32; 3]
+```
+
+Arrays have type `[T; N]`. We’ll talk about this `T` notation [in the generics
+section][generics].
+
+There’s a shorthand for initializing each element of an array to the same
+value. In this example, each element of `a` will be initialized to `0`:
+
+```rust
+let a = [0; 20]; // a: [i32; 20]
+```
+
+You can get the number of elements in an array `a` with `a.len()`:
+
+```rust
+let a = [1, 2, 3];
+
+println!("a has {} elements", a.len());
+```
+
+You can access a particular element of an array with *subscript notation*:
+
+```rust
+let names = ["Graydon", "Brian", "Niko"]; // names: [&str; 3]
+
+println!("The second name is: {}", names[1]);
+```
+
+Subscripts start at zero, like in most programming languages, so the first name
+is `names[0]` and the second name is `names[1]`. The above example prints
+`The second name is: Brian`. If you try to use a subscript that is not in the
+array, you will get an error: array access is bounds-checked at run-time. Such
+errant access is the source of many bugs in other systems programming
+languages.
+
+You can find more documentation for `array`s [in the standard library
+documentation][array].
+
+[array]: ../std/primitive.array.html
+
+# Slices
+
+A ‘slice’ is a reference to (or “view” into) an array. They are useful for
+allowing safe, efficient access to a portion of an array without copying. For
+example, you might want to reference just one line of a file read into memory.
+By nature, a slice is not created directly, but from an existing variable.
+Slices have a length, can be mutable or not, and in many ways behave like
+arrays:
+
+```rust
+let a = [0, 1, 2, 3, 4];
+let middle = &a[1..4]; // A slice of a: just the elements 1, 2, and 3
+```
+
+Slices have type `&[T]`. We’ll talk about that `T` when we cover
+[generics][generics].
+
+[generics]: generics.html
+
+You can find more documentation for `slices`s [in the standard library
+documentation][slice].
+
+[slice]: ../std/primitive.slice.html
+
+# `str`
+
+Rust’s `str` type is the most primitive string type. As an [unsized type][dst],
+it’s not very useful by itself, but becomes useful when placed behind a reference,
+like [`&str`][strings]. As such, we’ll just leave it at that.
+
+[dst]: unsized-types.html
+[strings]: strings.html
+
+You can find more documentation for `str` [in the standard library
+documentation][str].
+
+[str]: ../std/primitive.str.html
+
+# Tuples
+
+A tuple is an ordered list of fixed size. Like this:
+
+```rust
+let x = (1, "hello");
+```
+
+The parentheses and commas form this two-length tuple. Here’s the same code, but
+with the type annotated:
+
+```rust
+let x: (i32, &str) = (1, "hello");
+```
+
+As you can see, the type of a tuple looks just like the tuple, but with each
+position having a type name rather than the value. Careful readers will also
+note that tuples are heterogeneous: we have an `i32` and a `&str` in this tuple.
+In systems programming languages, strings are a bit more complex than in other
+languages. For now, just read `&str` as a *string slice*, and we’ll learn more
+soon.
+
+You can access the fields in a tuple through a *destructuring let*. Here’s
+an example:
+
+```rust
+let (x, y, z) = (1, 2, 3);
+
+println!("x is {}", x);
+```
+
+Remember [before][let] when I said the left-hand side of a `let` statement was more
+powerful than just assigning a binding? Here we are. We can put a pattern on
+the left-hand side of the `let`, and if it matches up to the right-hand side,
+we can assign multiple bindings at once. In this case, `let` "destructures,"
+or "breaks up," the tuple, and assigns the bits to three bindings.
+
+[let]: variable-bindings.html
+
+This pattern is very powerful, and we’ll see it repeated more later.
+
+There are also a few things you can do with a tuple as a whole, without
+destructuring. You can assign one tuple into another, if they have the same
+contained types and [arity]. Tuples have the same arity when they have the same
+length.
+
+[arity]: glossary.html#arity
+
+```rust
+let mut x = (1, 2); // x: (i32, i32)
+let y = (2, 3); // y: (i32, i32)
+
+x = y;
+```
+
+You can find more documentation for tuples [in the standard library
+documentation][tuple].
+
+[tuple]: ../std/primitive.tuple.html
+
+# Functions
+
+Functions also have a type! They look like this:
+
+```
+fn foo(x: i32) -> i32 { x }
+
+let x: fn(i32) -> i32 = foo;
+```
+
+In this case, `x` is a ‘function pointer’ to a function that takes an `i32` and
+returns an `i32`.
+
+You can find more documentation for function types [in the standard library
+documentation][fn].
+
+[fn]: ../std/primitive.fn.html