Is it not possible to use a user defined function in a loop?
I am not sure if I understand you properly, but in my opinion it is possible:
// Add your function to the volt compiler
$compiler->addFunction('square', function($arg) { return $arg . ' * ' . $arg; });
// call it in the view:
{% for i in 1..10 %} {{ i }} - {{ square(i) }}<br/> {% endfor %}
I tried something as Jacek says some days ago, but I always getted some error related with params.
But yeah, he is right about the syntax.