Anonymous Functions

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).


#![allow(unused)]
fn main() {
// 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 real closures, merely syntactic sugar):


#![allow(unused)]
fn main() {
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:


#![allow(unused)]
fn main() {
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 Real Closures

Remember: anonymous functions, though having the same syntax as Rust closures, are themselves not real closures.

In particular, they capture their execution environment via automatic currying (disabled via no_closure).