Functions: Anonymous Functions

Hack supports anonymous functions.

In the example below, the anonymous function, $f, evaluates to a function that returns the value of $x + 1.

$f = $x ==> $x + 1;

$two = $f(1); // result of 2

To create an anonymous function with more than one parameter, surround the parameter list with parentheses:

$f = ($x, $y) ==> $x + $y;

$three = $f(1, 2);  // result of 3

Anonymous functions pass by value, not by reference. This is also true for any object property passed to an anonymous function.

$x = 5;
$f = $x ==> $x + 1;

$six = $f($x); // pass by value

echo($six); // result of 6
echo("\n");
echo($x); // $x is unchanged; result of 5

If you need to mutate the reference, use the HSL Ref class.

Type Inference

Unlike named functions, type annotations are optional on anonymous functions. You can still add explicit types if you wish.

$f = (int $x): int ==> $x + 1;

HHVM will enforce type annotations if they are provided.

If typechecking cannot infer a type for a function, it will show an error, and you will need to provide a type. Adding explicit type annotations can also help the typechecker run faster.

Fat Arrow Syntax

==> defines an anonymous function in Hack. An anonymous function can be a single expression, or a block.

$f1 = $x ==> $x + 1;

$f2 = $x ==> { return $x + 1; };

Legacy PHP-Style Syntax

Hack also supports an anonymous function syntax similar to PHP. These are less flexible, so we recommend using fat arrow syntax.

$f = function($x) { return $x + 1; };

PHP-style lambdas require an explicit { ... } block.

PHP-style lambdas also require use to refer to enclosing variables. Fat arrow lambdas do not require this.

$y = 1;

$f = function($x) use($y) { return $x + $y; };

PHP-style lambdas can also specify parameter and return types.

$y = 1;

$f = function(int $x): int use($y) { return $x + $y; };

Note that this syntax is not the same as PHP 7 lambdas, which put use before the return type.