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Write the 'primitive types' section of TRPL #24253
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% Primitive Types | ||
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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. | ||
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# Booleans | ||
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Rust has a built in boolean type, named `bool`. It has two values, `true` and `false`: | ||
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```rust | ||
let x = true; | ||
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let y: bool = false; | ||
``` | ||
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A common use of booleans is in [`if` statements][if]. | ||
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[if]: if.html | ||
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You can find more documentation for `bool`s [in the standard library | ||
documentation][bool]. | ||
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[bool]: ../std/primitive.bool.html | ||
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# `char` | ||
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The `char` type represents a single Unicode scalar value. You can create `char`s | ||
with a single tick: (`'`) | ||
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```rust | ||
let x = 'x'; | ||
let two_hearts = '💕'; | ||
``` | ||
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Unlike some other languages, this means that Rust’s `char` is not a single byte, | ||
but four. | ||
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You can find more documentation for `char`s [in the standard library | ||
documentation][char]. | ||
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[char]: ../std/primitive.char.html | ||
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# Numeric types | ||
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Rust has a variety of numeric types in a few categories: signed and unsigned, | ||
fixed and variable, floating-point and integer. | ||
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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. | ||
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If a number literal has nothing to cause its type to be inferred, it defaults: | ||
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```rust | ||
let x = 42; // x has type i32 | ||
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let y = 1.0; // y has type f64 | ||
``` | ||
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Here’s a list of the different numeric types, with links to their documentation | ||
in the standard library: | ||
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* [i16](../std/primitive.i16.html) | ||
* [i32](../std/primitive.i32.html) | ||
* [i64](../std/primitive.i64.html) | ||
* [i8](../std/primitive.i8.html) | ||
* [u16](../std/primitive.u16.html) | ||
* [u32](../std/primitive.u32.html) | ||
* [u64](../std/primitive.u64.html) | ||
* [u8](../std/primitive.u8.html) | ||
* [isize](../std/primitive.isize.html) | ||
* [usize](../std/primitive.usize.html) | ||
* [f32](../std/primitive.f32.html) | ||
* [f64](../std/primitive.f64.html) | ||
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Let’s go over them by category: | ||
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## Signed and Unsigned | ||
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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`. | ||
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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 | ||
There was a problem hiding this comment. Choose a reason for hiding this commentThe reason will be displayed to describe this comment to others. Learn more. In this section, perhaps the mention of each primitive type could be linked to the documentation in the standard library? There was a problem hiding this comment. Choose a reason for hiding this commentThe reason will be displayed to describe this comment to others. Learn more. Ahhh yeah I forgot to do the master list for the numbers. |
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eight-bit signed number. | ||
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## Fixed size types | ||
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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. | ||
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## Variable sized types | ||
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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`. | ||
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## Floating-point types | ||
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Rust also two floating point types: `f32` and `f64`. These correspond to | ||
IEEE-754 single and double precision numbers. | ||
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# Arrays | ||
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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. | ||
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```rust | ||
let a = [1, 2, 3]; // a: [i32; 3] | ||
let mut m = [1, 2, 3]; // m: [i32; 3] | ||
``` | ||
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Arrays have type `[T; N]`. We’ll talk about this `T` notation [in the generics | ||
There was a problem hiding this comment. Choose a reason for hiding this commentThe reason will be displayed to describe this comment to others. Learn more. Could this elaborate on that There was a problem hiding this comment. Choose a reason for hiding this commentThe reason will be displayed to describe this comment to others. Learn more. 👍 |
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section][generics]. The `N` is a compile-time constant, for the length of the | ||
array. | ||
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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`: | ||
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```rust | ||
let a = [0; 20]; // a: [i32; 20] | ||
``` | ||
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You can get the number of elements in an array `a` with `a.len()`: | ||
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```rust | ||
let a = [1, 2, 3]; | ||
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println!("a has {} elements", a.len()); | ||
``` | ||
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You can access a particular element of an array with *subscript notation*: | ||
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```rust | ||
let names = ["Graydon", "Brian", "Niko"]; // names: [&str; 3] | ||
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println!("The second name is: {}", names[1]); | ||
``` | ||
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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. | ||
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You can find more documentation for `array`s [in the standard library | ||
documentation][array]. | ||
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[array]: ../std/primitive.array.html | ||
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# Slices | ||
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A ‘slice’ is a reference to (or “view” into) another data structure. 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: | ||
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```rust | ||
let a = [0, 1, 2, 3, 4]; | ||
let middle = &a[1..4]; // A slice of a: just the elements 1, 2, and 3 | ||
There was a problem hiding this comment. Choose a reason for hiding this commentThe reason will be displayed to describe this comment to others. Learn more. This isn't too early to introduce slicing syntax is it? I wonder if it could just stick to indexing here? There was a problem hiding this comment. Choose a reason for hiding this commentThe reason will be displayed to describe this comment to others. Learn more. Oh but then you couldn't create a slice, maybe not! There was a problem hiding this comment. Choose a reason for hiding this commentThe reason will be displayed to describe this comment to others. Learn more. yeah, let's leave that out for now, i'll cover it later |
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``` | ||
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Slices have type `&[T]`. We’ll talk about that `T` when we cover | ||
[generics][generics]. | ||
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[generics]: generics.html | ||
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You can find more documentation for `slices`s [in the standard library | ||
documentation][slice]. | ||
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[slice]: ../std/primitive.slice.html | ||
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# `str` | ||
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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. | ||
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[dst]: unsized-types.html | ||
[strings]: strings.html | ||
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You can find more documentation for `str` [in the standard library | ||
documentation][str]. | ||
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[str]: ../std/primitive.str.html | ||
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# Tuples | ||
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A tuple is an ordered list of fixed size. Like this: | ||
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```rust | ||
let x = (1, "hello"); | ||
``` | ||
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The parentheses and commas form this two-length tuple. Here’s the same code, but | ||
with the type annotated: | ||
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```rust | ||
let x: (i32, &str) = (1, "hello"); | ||
``` | ||
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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. | ||
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You can access the fields in a tuple through a *destructuring let*. Here’s | ||
an example: | ||
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```rust | ||
let (x, y, z) = (1, 2, 3); | ||
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println!("x is {}", x); | ||
``` | ||
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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. | ||
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[let]: variable-bindings.html | ||
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This pattern is very powerful, and we’ll see it repeated more later. | ||
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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. | ||
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[arity]: glossary.html#arity | ||
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```rust | ||
let mut x = (1, 2); // x: (i32, i32) | ||
let y = (2, 3); // y: (i32, i32) | ||
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x = y; | ||
``` | ||
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You can find more documentation for tuples [in the standard library | ||
documentation][tuple]. | ||
There was a problem hiding this comment. Choose a reason for hiding this commentThe reason will be displayed to describe this comment to others. Learn more. Should tuple indexing notation be mentioned here? (or is it covered later?) There was a problem hiding this comment. Choose a reason for hiding this commentThe reason will be displayed to describe this comment to others. Learn more. So originally it was here, but it felt weird to go so heavily into this particular type as compared to other primitive types, so I was thinking about putting it on the tuple primtive page. Thoughts? There was a problem hiding this comment. Choose a reason for hiding this commentThe reason will be displayed to describe this comment to others. Learn more. Sounds good to me! There was a problem hiding this comment. Choose a reason for hiding this commentThe reason will be displayed to describe this comment to others. Learn more. already there! |
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[tuple]: ../std/primitive.tuple.html | ||
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# Functions | ||
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Functions also have a type! They look like this: | ||
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``` | ||
fn foo(x: i32) -> i32 { x } | ||
let x: fn(i32) -> i32 = foo; | ||
``` | ||
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In this case, `x` is a ‘function pointer’ to a function that takes an `i32` and | ||
returns an `i32`. |
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Perhaps this could link to something like the wikipedia article for two's complement?
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I have complex feels regarding that. Hrm.
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Oh no worries! It's also something that's super easy to google :)
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I was also considering writing a section in the glossary about it, maybe I'll do that in the future :)