design-patterns.md

Basic Design Patterns

.clone() before Lifetime Annotations

• As a beginner, use .clone() to overcome compiler struggle.
• It is alright! Refactor later.

String before &str

• Use "owned" types before references.
• It is alright! Refactor later.

String concatenation: Use format!()

• Owned type String can be generated easily.
• let s: String = format!("No fear from {}", "Rust Strings")

Clippy is your friend in linting

• A collection of lints to catch common mistakes and improve your Rust code.
• Installation: rustup component add clippy
• Run: cargo clippy
• Documentation: https://rust-lang.github.io/rust-clippy/stable/index.html

Pattern: From<T>, Into<T>

Conversion of one Type into another.

If X is From<T>, then T is Into<X> automatically.

The usage depends on the context.

Pattern: From<T>, Into<T> - Example

fn main() {
    let string = String::from("string slice");
    let string2: String = "string slice".into();
}

Pattern: What does ? do?

use std::fs::File;
use std::io::{self, Write};

enum MyError {
    FileWriteError,
}

impl From<io::Error> for MyError {
    fn from(e: io::Error) -> MyError {
        MyError::FileWriteError
    }
}

fn write_to_file_using_q() -> Result<(), MyError> {
    let mut file = File::create("my_best_friends.txt")?;
    file.write_all(b"This is a list of my best friends.")?;
    println!("I wrote to the file");
    Ok(())
}
// This is equivalent to:
fn write_to_file_using_match() -> Result<(), MyError> {
    let mut file = File::create("my_best_friends.txt")?;
    match file.write_all(b"This is a list of my best friends.") {
        Ok(v) => v,
        Err(e) => return Err(From::from(e)),
    }
    println!("I wrote to the file");
    Ok(())
}

fn main() {}

Pattern: AsRef<T>

Reference-to-reference-conversion. Indicates that a type can easily produce references to another type.

Pattern: AsRef<T> - Example

use std::fs::File;
use std::path::Path;
use std::path::PathBuf;

fn main() {
    open_file(&"test");
    let path_buf = PathBuf::from("test");
    open_file(&path_buf);
}

fn open_file<P: AsRef<Path>>(p: &P) {
    let path = p.as_ref();
    let file = File::open(path);
}

Pattern: Constructor new()

• No constructors, but there is a convention.
• An associated function to construct new "instances".
• Use Default trait. Try using #[derive(Default)] first.

pub struct Stuff {
    value: i64,
}

impl Stuff {
    /// constructor by convention
    fn new(value: i64) -> Self {
        Self { value: value }
    }
}

Pattern: NewType

• Use Rust type system to convey meaning to the user.
• Especially for Types that should be similar to other Types.
• Also used to impl external Traits on external Types

struct MyString(String);

impl MyString {
    //... my implementations for MyString
}

Pattern: Extending external Types

• Recall that at least one of Trait or Type should be local to impl.
• This pattern allows you to extend external Type using a local Trait.

trait VecExt {
    fn magic_number(&self) -> usize;
}

impl<T> VecExt for Vec<T> {
    fn magic_number(&self) -> usize {
        42
    }
}

fn main() {
    let v = vec![1, 2, 3, 4, 5];
    println!("Magic Number = {}", v.magic_number());
}

Pattern: Narrowing variable's scope

• Shadowing allows you to redefine a variable with let keyword again.
• Use it to get the inner Type, say in Option.
• Use it to your advantage to make variable immutable after it's served its purpose.

// Get the inner type from Option
let array = [1, 2, 3, 4];
let item = array.get(1);
if let Some(item) = item { 
    println!("{:?}", item);
}

// Use shadowing to make the variable immutable outside of 
// where it needs to be mutable
let mut data = 42;
// change the data 
data += 1;
// Shadow using `let` again
let data = data; 
// data is immutable from now on