fun is a special function used to create a "pointer" to a function in a
function fun( string $s, )
The typechecker disallows using strings as functions; you must instead use
fun() to make sure the typechecker looks up the function signature and
returns a proper function type.
<?hh $a = [1, 2, 3]; $c = 'count'; $c($a); // Type error: cannot call a string. $c = fun('count'); $c($a); // Legal: by using fun(), $c is now a callable with the right type.
$sFunction to look up. Must be a constant string.
A- callback which will call
$swhen invoked, but has the proper Hack function signature.