Hack supports anonymous functions.

$f = $x ==> $x + 1; $two = $f(1);

Type Inference

Unlike named functions, type anotations 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; };