rhai/doc/src/language/fn-anon.md

Anonymous Functions

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Sometimes it gets tedious to define separate functions only to dispatch them via single [function pointers]. This scenario is especially common when simulating object-oriented programming ([OOP]).

// Define object
let obj = #{
    data: 42,
    increment: Fn("inc_obj"),   // use function pointers to
    decrement: Fn("dec_obj"),   // refer to method functions
    print: Fn("print_obj")
};

// Define method functions one-by-one
fn inc_obj(x) { this.data += x; }
fn dec_obj(x) { this.data -= x; }
fn print_obj() { print(this.data); }

The above can be replaced by using anonymous functions which have the same syntax as Rust's closures (but they are NOT closures, merely syntactic sugar):

let obj = #{
    data: 42,
    increment: |x| this.data += x,          // one-liner
    decrement: |x| this.data -= x,
    print_obj: || { print(this.data); }     // full function body
};

The anonymous functions will be hoisted into separate functions in the global namespace. The above is equivalent to:

let obj = #{
    data: 42,
    increment: Fn("anon_fn_1000"),
    decrement: Fn("anon_fn_1001"),
    print: Fn("anon_fn_1002")
};

fn anon_fn_1000(x) { this.data += x; }
fn anon_fn_1001(x) { this.data -= x; }
fn anon_fn_1002() { print this.data; }

WARNING - NOT Closures

Remember: anonymous functions, though having the same syntax as Rust closures, are themselves not closures. In particular, they do not capture their running environment. They are more like Rust's function pointers.