# 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
};``````

NOT real closures

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.