dev-notes/Rust/Rust.md

Rust

Compiling and running

rustc file.rs  # compilation
file.exe  # running

Basics

use <module>;  // bring a type into scope

fn main() {  //program entry point
    // code here
}

Result types

The Result types are enumerations, often referred to as enums. An enumeration is a type that can have a fixed set of values, and those values are called the enum's variants.

For Result, the variants are Ok or Err.
The Ok variant indicates the operation was successful, and inside Ok is the successfully generated value.
The Err variant means the operation failed, and Err contains information about how or why the operation failed.

The purpose of the Result types is to encode error-handling information.

Rust has a number of types named Result in its standard library: a generic Result as well as specific versions for submodules.

Standard Output

// macro (funcs have no "!")
println!("Value: {}", value);  // {} is a placeholder for a value or variable
println!("Values: {1}, {0}", value1, value2);  // use index to print values
println!("Num: {:<total_digits>.<decimal_digits>}", 10);
println!("Num: {:0<total>.<decimal>}", 10);  // prefix num with zeroes

println!("{:b}", 0b11011);  // print as bits

print!();

Standard Input

use io;

let mut buffer = String::new();

io::stdin()  // read line from stdin
    .readline(&mut buffer)  // put in into the string variable
    .expect("Error Message");  // in case of errors return an error message

let var: i32 = buffer.trim().parse()  // returns a Result enum
let var = buffer.trim().parse::<i32>()  // returns a Result enum

The :: syntax in the ::new line indicates that new is an associated function of the String type.
An associated function is implemented on a type rather than on a particular instance of the type. Some languages call this a static method.

Variables & Mutability

By default variables are immutable.

let var1 = value;  // immutable var init
let var2: Type = value;  // explicit type annotation

let mut var3 = value;  // mutable var init
let mut var4: Type = value;  // explicit type annotation

const CONSTANT_NAME: type = value;  // constant must have the type annotation

Shadowing

It's possible declare a new variable with the same name as a previous variable, and the new variable shadows the previous variable.
By using let, it's possible to perform a few transformations on a value but have the variable be immutable after those transformations have been completed.

The other difference between mut and shadowing is that because we're effectively creating a new variable when we use the let keyword again,
we can change the type of the value but reuse the same name.

let x: u32 = 10;
let x: i32 = 11;  // shadowing

Data Types

Integer Types

Length	Signed	Unsigned
8-bit	i8	u8
16-bit	i16	u16
32-bit	i32	u32
64-bit	i64	u64
128-bit	i128	u128
architecture	isize	usize

Floating-Point Types

Rust also has two primitive types for floating-point numbers, which are numbers with decimal points.
Rust's floating-point types are f32 and f64, which are 32 bits and 64 bits in size, respectively.
The default type is f64 because on modern CPUs it's roughly the same speed as f32 but is capable of more precision.

Numeric Operators

Operator	Operation
>	Greater than
>=	Greater or Equal to
<	Less than
<=	Less or Equal to
==	Equals
!=	Not Equals

Comparison Operators

Operator	Operation
+	Addition
-	Subtraction
*	Multiplication
/	Division
%	Modulo

Boolean Types

Boolean types in Rust have two possible values: true and false.
Booleans are one byte in size. The Boolean type in Rust is specified using bool.

Bitwise Operators

Operator	Operation
&	AND
`	`
&&	SHORT-CIRCUIT AND
`
^	XOR
!	NOT
<<	LEFT SHIFT
>>	RIGHT SHIFT

Character Types

Rust's char type is the language's most primitive alphabetic type.

Rust's char type is four bytes in size and represents a Unicode Scalar Value: range from U+0000 to U+D7FF and U+E000 to U+10FFFF inclusive.

let c: char = 'C';  // SINGLE QUOTES
let c: char = '\u{261D}';  // Unicode Code Point U+261D

String Types

let s = String::new();  // create empty string
let s = String::from("string literal");  // construct string from literal

s.push_str(""); // appending string literals

Tuple Types

A tuple is a general way of grouping together a number of values with a variety of types into one compound type.
Tuples have a fixed length: once declared, they cannot grow or shrink in size.

let tup: (i32, f64, u8) = (500, 6.4, 1);
let tup = (500, 6.4, 1);

let (x, y, z) = tup;  // tuple deconstruction (unpacking)

tup.0 = value;  // member access & update (mut be mutable)

Array Types

Every element of an array must have the same type. Arrays in Rust have a fixed length, like tuples.
An array isn't as flexible as the vector type, though. A vector is a similar collection type provided by the standard library that is allowed to grow or shrink in size.

let array = [0, 1, 2, 3, 4];
let array: [Type; length] = [...];
let array: [value; length];  // repeat expression (same as python's [value] * length)

let index: usize = <index_value>;  // indexes and ranges must be of type usize
array[index] = value;  // member access and update (must be mutable)

let matrix = [
    [0 ,1, 2],
    [3, 4, 5]
]

let matrix: [[Type, length]; length]; = [[...], [...], ...]

matrix[row][column];

Slice Types

Slices allows to reference a contiguous sequence of elements in a collection rather than the whole collection. Slices don't take ownership.

A string slice (&str) length is in bytes, can cause panic if it happens inside a character. This can happen because Unicode characters can take up to 4 bytes.

&String can be used in place of a &str trough String Coercion. The &String gets converted to a &str that borrows the entire string. The reverse is not possible since the slice lacks some information about the String.

NOTE: When working with functions is easier to always expect a &str instead of a &String.

let s = String::from("string literal");
let slice: &str = &s[start..end];

let a = [0, 1, 2, 3, 4, 5];
let slice: &[i32] =  &a[start..end];

sequence[start..]  // slice to end of sequence
sequence[..end]  // slice from start to end (excluded)

Functions

Rust code uses snake_case as the conventional style for function and variable names.

Function definitions in Rust start with fn and have a set of parentheses after the function name.
The curly brackets tell the compiler where the function body begins and ends.

Rust doesn't care where the functions are defined, only that they're defined somewhere.

fn func(param: Type) {  // parameters MUST have the Type annotation
    // code here
}

fn func() -> Type {  // -> specifies the return type
    // code here
}

fn func() {
    value  // returns value
}
// same as
fn func() {
    return value;
}

fn func() {
    return (value1, value2, ...);  // return multiple values with tuples
}

Control Flow

if - else if - else

if condition {
    // code here
} else if condition {
    // code here
} else {
    // code here
}

let if

let var = if condition { value } else { value };  // returned types must be the same

loop

loop {  // loop's forever if not explicitly stopped
    // code here

    if condition {
        break returned_value
    }
}

while

while condition {  // runs while condition is true
    // code here
}

for

for item in sequence.iter() { }
for item in sequence.iter_mut() { }  // iterate over mutable items

// item is a reference to the value in the sequence, use & and * to access the value or address

for i in (start..end) {  // (start..stop) is like python's range(start, stop)
    // code here
}

Ownership

Ownership is Rust's most unique feature, and it enables Rust to make memory safety guarantees without needing a garbage collector.

All programs have to manage the way they use a computer's memory while running.
Some languages have garbage collection that constantly looks for no longer used memory as the program runs; in other languages, the programmer must explicitly allocate and free the memory.
Rust uses a third approach: memory is managed through a system of ownership with a set of rules that the compiler checks at compile time.
None of the ownership features slow down your program while it's running.

Stack & Heap

Both the stack and the heap are parts of memory that are available to your code to use at runtime, but they are structured in different ways.

The stack stores values in the order it gets them and removes the values in the opposite order. This is referred to as last in, first out.
Adding data is called pushing onto the stack, and removing data is called popping off the stack.

All data stored on the stack must have a known, fixed size. Data with an unknown size at compile time or a size that might change must be stored on the heap instead.

The heap is less organized: when you put data on the heap, you request a certain amount of space. The memory allocator finds an empty spot in the heap that is big enough, marks it as being in use, and returns a pointer, which is the address of that location. This process is called allocating on the heap and is sometimes abbreviated as just allocating.

Pushing to the stack is faster than allocating on the heap because the allocator never has to search for a place to store new data; that location is always at the top of the stack.
Comparatively, allocating space on the heap requires more work, because the allocator must first find a big enough space to hold the data and then perform bookkeeping to prepare for the next allocation.

Accessing data in the heap is slower than accessing data on the stack because you have to follow a pointer to get there. Contemporary processors are faster if they jump around less in memory.

Keeping track of what parts of code are using what data on the heap, minimizing the amount of duplicate data on the heap, and cleaning up unused data on the heap so you don't run out of space are all problems that ownership addresses.

Ownership Rules

• Each value in Rust has a variable that's called its owner.
• There can only be one owner at a time.
• When the owner goes out of scope, the value will be dropped.

Moving, Cloning & Copying data

A "shallow copy" of a variable allocated on the heap causes the original variable goes out of scope and only the "copy" remains (MOVING).
A "deep copy" (var.clone()) makes a copy of the data in the new variable without make the original fall out of scope (CLONING).

When a variable goes out of scope, Rust calls the special function drop, where the code to return/free the memory is located.

Rust has a special annotation called the Copy trait that it's placeable on types that are stored on the stack.
If a type has the Copy trait, an older variable is still usable after assignment.

Rust won't allow to annotate a type with the Copy trait if the type, or any of its parts, has implemented the Drop trait.

let s = String::new()
let t = s;  // shallow copy, s is now out of scope
let u = t.clone();  // deep copy, t is still valid

let n: i32 = 1;
let x = n;  // x holds a COPY of the VALUE of n

Ownership & Functions

The semantics for passing a value to a function are similar to those for assigning a value to a variable. Passing a variable to a function will move or copy, just as assignment does.

fn main() {
    let s = String::from("hello");  // s comes into scope

    takes_ownership(s);             // s's value moves into the function and so is no longer valid here

    let x = 5;                      // x comes into scope

    makes_copy(x);                  // x would move into the function, but i32 is Copy, so it's okay to still use x afterward

} // Here, x goes out of scope, then s. But because s's value was moved, nothing special happens.

fn takes_ownership(some_string: String) { // some_string comes into scope
    println!("{}", some_string);
} // Here, some_string goes out of scope and `drop` is called. The backing memory is freed.

fn makes_copy(some_integer: i32) { // some_integer comes into scope
    println!("{}", some_integer);
} // Here, some_integer goes out of scope. Nothing special happens.

Return Values & Scope

Returning values can also transfer ownership.

fn main() {
    let s1 = gives_ownership();         // gives_ownership moves its return value into s1

    let s2 = String::from("hello");     // s2 comes into scope

    let s3 = takes_and_gives_back(s2);  // s2 is moved into takes_and_gives_back, which also moves its return value into s3
} // Here, s3 goes out of scope and is dropped. s2 goes out of scope but was moved, so nothing happens. s1 goes out of scope and is dropped.

fn gives_ownership() -> String {             // gives_ownership will move its return value into the function that calls it

    let some_string = String::from("hello"); // some_string comes into scope

    some_string                              // some_string is returned and moves out to the calling function
}

// takes_and_gives_back will take a String and return one
fn takes_and_gives_back(a_string: String) -> String { // a_string comes into scope

    a_string  // a_string is returned and moves out to the calling function
}

References & Borrowing

Mutable references have one big restriction: you can have only one mutable reference to a particular piece of data in a particular scope.

The benefit of having this restriction is that Rust can prevent data races at compile time.
A data race is similar to a race condition and happens when these three behaviors occur:

• Two or more pointers access the same data at the same time.
• At least one of the pointers is being used to write to the data.
• There's no mechanism being used to synchronize access to the data.

fn borrow(var: &Type) {  // &Type indicates that the var is a reference
    // here var cannot be modified
} // when var goes out of scope it doesn't get dropped because the scope didn't own it

borrow(&variable);  // &variable creates a reference to variable but does not take ownership

fn borrow2(var: &mut Type) {
    // here var can be modified
}

&variable;  // borrow a value from a variable (returns reference)
*variable;  // dereference value (access value pointed by the reference)

Structs

A struct, or structure, is a custom data type that allows to name and package together multiple related values that make up a meaningful group.

To define a struct enter the keyword struct and name the entire struct.
A struct's name should describe the significance of the pieces of data being grouped together.
Then, inside curly brackets, define the names and types of the pieces of data, which we call fields.

struct Struct {
    field: Type,
    ...
}

{ x: Type, y: Type }  // anonymous struct

To use a struct after defining it, create an instance of that struct by specifying concrete values for each of the fields.

let mut var = Struct {
    field: value,
    ...
};

fn build_struct(param: Type, ...) -> Struct {
    // the constructed struct is returned since it's the last expression
    Struct {
        field: param,
        ...
    }
}

Field Init Shorthand

fn build_struct(field: Type, ...) -> Struct {
    // the constructed struct is returned since it's the last expression
    Struct {
        field,  // shortened form since func param is named as the struct's field
        ...
    }
}

var.field = value;  // member access

Note: the entire instance must be mutable; Rust doesn't allow to mark only certain fields as mutable.

Struct Update Syntax

let struct1 = Struct {
    field1: value,
    field2: value,
    ...
}
// same as
let struct2 = Struct {
    field1: other_value,
    ..struct1  // all remaining fields have the same values of struct1
}

Tuple Structs

Use Tuple Structs to create different types easily.
To define a tuple struct, start with the struct keyword and the struct name followed by the types in the tuple.

struct Point(i32, i32, i32);
struct Color(i32, i32, i32);

let origin = Point(0, 0, 0);

Struct Printing

#[derive(Debug)]  // inherit the debug trait
struct StructName
{
    field: value,
    ...
}

let s: Struct = { /* valorization */};
println!("{:?}", s)  // debug output: { field: value, ... }

Methods & Associated Functions

struct Struct
{
    field: value,
    ...
}

impl Struct
{
    fn method(&self, arg: Type) -> Type { }
    fn method(&mut self, arg: Type) -> Type { }  // able to modify instance
    fn associated_func(arg: Type) -> Type { }  // "static" method
}

let s: Struct = { /* valorization */};
s.method(arg);  // use struct method

Struct::associated_func(arg);

Generic Structs & Methods

Generic Data Types are abstract stand-ind for concrete data types or other properties.
They can be used with structs, functions, methods, etc.

struct GenericStruct<T, U> {
    T generic_field,
    U generic_field,
    ...
}

impl<T, U> GenericStruct<T, U> {
    fn generic_method<T>() -> Type { }
    fn generic_method<U>() -> Type { }
}

// implementation for specific types
impl GenericStruct<Type, Type> {
    fn method() -> Type { }
}

fn generic_func<T>() -> Type { }
fn generic_func<T>() -> &T { }  // T could be heap-based type, returning reference is always valid

fn generic<T: Trait>() -> Type { }  // use generic constraint

Enums

// enum definition
enum Enum
{
    Variant1(Type, ...),  // each variant can have different types (even structs and enums) and amounts of associated data
    Variant2,
    ...
}

// value assignment
let e: Enum = Enum::Variant2;
let e: Enum = Enum::Variant1(arg, ...);  // variant w/ data

// methods on enum
impl Enum
{
    fn method(&self, arg: Type) -> Type {}
}

Option enum

The Option type is used in many places because it encodes the very common scenario in which a value could be something or it could be nothing. Expressing this concept in terms of the type system means the compiler can check whether you've handled all the cases you should be handling; this functionality can prevent bugs that are extremely common in other programming languages.

he Option<T> enum is so useful that it's even included in the prelude; you don't need to bring it into scope explicitly.
In addition, so are its variants: you can use Some and None directly without the Option:: prefix.

// std implementation
enum Option<T> {
    Some(T),
    None
}

NOTE: When None is used the type of Option<T> must be specified, because the compiler can't infer the type that the Some variant will hold by looking only at a None value.

Match Expression + Comparing

A match expression is made up of arms.
An arm consists of a pattern and the code that should be run if the value given to the beginning of the match expression fits that arm's pattern.
Rust takes the value given to match and looks through each arm's pattern in turn.

NOTE: match arms must be exhaustive for compilation.

enum Enum {
    Variant1,
    Variant2,
    Variant3(value),
    ...
}

// match expression on enum
fn match_variant(e: Enum) {
    match e {
        Enum::Variant1 => <expr>,
        Enum::Variant2 => { /* code block */ },
        Enum::Variant3(value) => state,
        _ => ()  // () is unit value
    }
}

Collections

Vector

Vectors allow to store more than one value in a single data structure that puts all the values next to each other in memory. Vectors can only store values of the same type.
Like any other struct, a vector is freed when it goes out of scope. When the vector gets dropped, all of its contents are also dropped.

let v: Vec<Type> = Vec<Type>::new();  // empty vec init
let mut v: vec![item1, item2, ...];  // vec init (type inferred)

v.push(item);  // add elements to vector

// element access
v.get(index);  // get method (returns Option<&T>)
&v[index];  // index syntax  (returns reference, panic on index out of bounds)

// iterate over mutable references to each element in a mutable vector in order to make changes to all the elements
for i in mut &v {
    *i = value;  // dereference and modify value
}

A vector can hold different types if those type are variants of the same enum. It's also possible to use trait objects.

enum Enum {
    Int(i32),
    Float(f64),
    Text(String)
}

let v = vec![
    Enum::Int(2),
    Enum::Float(3.14),
    Enum::Text("TEST")
];

Files

Reading Files

use std::fs;

let contents: Vec<u8> = fs::read("path/to/file").unwrap_or_default();
let contents: String = fs::read_to_string("path/to/file").unwrap_or_default();

contents.lines(); // iterator over text lines

Writing Files

use std::fs;
use std::io::Write;  // write trait
// or
use std::io::prelude::*;

let contents: [u8] = /* */;
let contents = String::from(/* */);

fs::write("path/to/file", contents);

let mut file = fs::OpenOptions::new().append(true).open("path/to/file").unwrap();
file.write(b"appended text");  // write wants an [u8]