Anonymous Functions
Many functions in the standard API expect function pointer as parameters.
For example:
// Function 'double' defined here - used only once
fn double(x) { 2 * x }
// Function 'square' defined here - again used only once
fn square(x) { x * x }
let x = [1, 2, 3, 4, 5];
// Pass a function pointer to 'double'
let y = x.map(double);
// Pass a function pointer to 'square' using Fn(...) notation
let z = y.map(Fn("square"));
Sometimes it gets tedious to define separate functions only to dispatch them via single function pointers – essentially, those functions are only ever called in one place.
This scenario is especially common when simulating object-oriented programming ([OOP]).
// Define functions one-by-one
fn obj_inc(x, y) { this.data += x * y; }
fn obj_dec(x) { this.data -= x; }
fn obj_print() { print(this.data); }
// Define object
let obj = #{
data: 42,
increment: obj_inc, // use function pointers to
decrement: obj_dec, // refer to method functions
print: obj_print
};
Syntax
Anonymous functions have a syntax similar to Rust’s closures (they are not the same).
|
param 1,
param 2,
…,
param n|
statement
|
param 1,
param 2,
…,
param n| {
statements…}
No parameters:
||
statement
|| {
statements…}
Rewrite Using Anonymous Functions
The above can be rewritten using anonymous functions.
let x = [1, 2, 3, 4, 5];
let y = x.map(|x| 2 * x);
let z = y.map(|x| x * x);
let obj = #{
data: 42,
increment: |x, y| this.data += x * y, // one statement
decrement: |x| this.data -= x, // one statement
print_obj: || {
print(this.data); // full function body
}
};
This de-sugars to:
// Automatically generated...
fn anon_fn_0001(x) { 2 * x }
fn anon_fn_0002(x) { x * x }
fn anon_fn_0003(x, y) { this.data += x * y; }
fn anon_fn_0004(x) { this.data -= x; }
fn anon_fn_0005() { print(this.data); }
let x = [1, 2, 3, 4, 5];
let y = x.map(anon_fn_0001);
let z = y.map(anon_fn_0002);
let obj = #{
data: 42,
increment: anon_fn_0003,
decrement: anon_fn_0004,
print: anon_fn_0005
};
Remember: though having the same syntax as Rust closures, anonymous functions are themselves NOT real closures.
In particular, they capture their execution environment via automatic currying.