🔑 KeyPaths & CasePaths in Rust

Key paths and case paths provide a safe, composable way to access and modify nested data in Rust. Inspired by Swift’s KeyPath / CasePath system, this crate lets you work with struct fields and enum variants as first-class values.

✨ Features

• ✅ ReadableKeyPath → safely read struct fields.
• ✅ WritableKeyPath → safely read/write struct fields.
• ✅ EnumKeyPath (CasePaths) → extract and embed enum variants.
• ✅ Composable → chain key paths together(Upcoming).
• ✅ Iterable → iterate or mutate values across collections.
• ✅ Macros → concise readable_keypath!, writable_keypath!, enum_keypath!.

📦 Installation

[dependencies]
key_paths_core = "0.6"

🚀 Examples - Go to latest examples directory docs will be updated later

1. CasePaths with Enums

use key_paths_core::KeyPaths;
#[derive(Debug)]
enum Payment {
    Cash { amount: u32 },
    Card { number: String, cvv: String },
}

fn main() {
    let kp = KeyPaths::writable_enum(
        |v| Payment::Cash { amount: v },
        |p: &Payment| match p {
            Payment::Cash { amount } => Some(amount),
            _ => None,
        },
        |p: &mut Payment| match p {
            Payment::Cash { amount } => Some(amount),
            _ => None,
        },

    );

    let mut p = Payment::Cash { amount: 10 };

    println!("{:?}", p);

    if let Some(v) = kp.get_mut(&mut p) {
        *v = 34
    }
    println!("{:?}", p);
}

2. Readable KeyPaths - helper macros wip

use key_paths_core::KeyPaths;

#[derive(Debug)]
struct Size {
    width: u32,
    height: u32,
}

#[derive(Debug)]
struct Rectangle {
    size: Size,
    name: String,
}

fn main() {
    let mut rect = Rectangle {
        size: Size {
            width: 30,
            height: 50,
        },
        name: "MyRect".into(),
    };

    let width_direct = KeyPaths::readable(|r: &Rectangle| &r.size.width);
    println!("Width: {:?}", width_direct.get(&rect));
}

3. Writable KeyPaths - helper macros wip

use key_paths_core::KeyPaths;

#[derive(Debug)]
struct Size {
    width: u32,
    height: u32,
}
#[derive(Debug)]
struct Rectangle {
    size: Size,
    name: String,
}
fn main() {
    let mut rect = Rectangle {
        size: Size {
            width: 30,
            height: 50,
        },
        name: "MyRect".into(),
    };
    let width_mut = KeyPaths::writable(
        |r: &mut Rectangle| &mut r.size.width,
    );
    // Mutable
    if let Some(hp_mut) = width_mut.get_mut(&mut rect) {
        *hp_mut += 50;
    }
    println!("Updated rectangle: {:?}", rect);
}

4. Composability and failablity

use key_paths_core::KeyPaths;

#[derive(Debug)]
struct Engine {
   horsepower: u32,
}
#[derive(Debug)]
struct Car {
   engine: Option<Engine>,
}
#[derive(Debug)]
struct Garage {
   car: Option<Car>,
}

fn main() {
   let mut garage = Garage {
       car: Some(Car {
           engine: Some(Engine { horsepower: 120 }),
       }),
   };

   let kp_car = KeyPaths::failable_writable(|g: &mut Garage| g.car.as_mut());
   let kp_engine = KeyPaths::failable_writable(|c: &mut Car| c.engine.as_mut());
   let kp_hp = KeyPaths::failable_writable(|e: &mut Engine| Some(&mut e.horsepower));

   // Compose: Garage -> Car -> Engine -> horsepower
   let kp = kp_car.compose(kp_engine).compose(kp_hp);

   println!("{garage:?}");
   if let Some(hp) = kp.get_mut(&mut garage) {
       *hp = 200;
   }

   println!("{garage:?}");
}

4. Mutablity

use key_paths_core::KeyPaths;

#[derive(Debug)]
struct Size {
   width: u32,
   height: u32,
}
#[derive(Debug)]
enum Color {
   Red,
   Green,
   Blue,
   Other(RGBU8),
}
#[derive(Debug)]
struct RGBU8(u8, u8, u8);

#[derive(Debug)]
struct ABox {
   name: String,
   size: Size,
   color: Color,
}
#[derive(Debug)]
struct Rectangle {
   size: Size,
   name: String,
}
fn main() {
   let mut a_box = ABox {
       name: String::from("A box"),
       size: Size {
           width: 10,
           height: 20,
       },
       color: Color::Other(
           RGBU8(10, 20, 30)
       ),
   };

   let color_kp: KeyPaths<ABox, Color> = KeyPaths::failable_writable(|x: &mut ABox| Some(&mut x.color));
   let case_path = KeyPaths::writable_enum(
       {
           |v| Color::Other(v)
       },
       |p: &Color| match p {
           Color::Other(rgb) => Some(rgb),
           _ => None,
       },
       |p: &mut Color| match p {
           Color::Other(rgb) => Some(rgb),
           _ => None,
       },

   );
   
   println!("{:?}", a_box);
   let color_rgb_kp = color_kp.compose(case_path);
   if let Some(value) = color_rgb_kp.get_mut(&mut a_box) {
       *value = RGBU8(0, 0, 0);
   }
   println!("{:?}", a_box);
}
/*
ABox { name: "A box", size: Size { width: 10, height: 20 }, color: Other(RGBU8(10, 20, 30)) }
ABox { name: "A box", size: Size { width: 10, height: 20 }, color: Other(RGBU8(0, 0, 0)) }
*/

🔗 Helpful Links & Resources

• 📘 type-safe property paths
• 📘 Swift KeyPath documentation
• 📘 Elm Architecture & Functional Lenses
• 📘 Rust Macros Book
• 📘 Category Theory in FP (for intuition)

💡 Why use KeyPaths?

• Avoids repetitive match / . chains.
• Encourages compositional design.
• Plays well with DDD (Domain-Driven Design) and Actor-based systems.
• Useful for reflection-like behaviors in Rust (without unsafe).

🛠 Roadmap

• compose support for combining multiple key paths.
• Derive macros for automatic KeyPath generation (Upcoming).
• Nested struct & enum traversal.
• Optional chaining (User?.profile?.name) with failable.

📜 License

• Mozilla Public License 2.0