How to Write Fluent APIs (Method Chaining) in Rust

You write fluent APIs in Rust by defining methods that return Self or &mut Self to enable chaining. Define a struct, implement methods that take &mut self and return &mut Self, then call them sequentially on the same instance.

struct Builder {
    value: i32,
}

impl Builder {
    fn new() -> Self {
        Builder { value: 0 }
    }

    fn add(mut self, n: i32) -> Self {
        self.value += n;
        self
    }

    fn build(self) -> i32 {
        self.value
    }
}

fn main() {
    let result = Builder::new().add(10).add(20).build();
    println!("{result}"); // 30
}

Fluent APIs let you chain method calls together like a sentence instead of writing separate lines for each step. You achieve this by making each method return the object itself so the next method can be called immediately. It is like a command chain where each step passes the work to the next one without stopping.