Get Started

Ownership & Borrowing

Sailfin uses move-by-default semantics to manage resources without a garbage collector.

Move Semantics

When you assign a value, ownership moves:

let data = Vec.new();
let moved = data;        // ownership moves to `moved`
// print.info(data);     // ERROR: use after move

Borrowing

To access a value without taking ownership, use borrows:

fn print_length(items: &Vec<Int>) ![io] {
    print.info("Length: {{items.len()}}");
    // items is borrowed — caller still owns it
}

fn add_item(items: &mut Vec<Int>) {
    items.push(42);
    // exclusive mutable access
}

Borrow Rules

  • You can have many &T (shared/read-only) borrows at once
  • You can have one &mut T (exclusive/mutable) borrow at a time
  • You cannot have &T and &mut T at the same time

Affine and Linear Types

For resources that need stricter guarantees:

// Affine<T>: may be dropped, but cannot be duplicated
let handle: Affine<FileHandle> = open("data.txt");
// let copy = handle;  // ERROR: Affine values can't be copied

// Linear<T>: must be consumed exactly once
let token: Linear<AuthToken> = mint_token();
// Dropping token without consuming it is a compile error
consume_token(token);  // OK: consumed

Next Steps