Twig function/filter with no input? - twig

Question

I'm using Slim 3 and Slim Twig-View. I want to add a Twig function (or filter, not sure what is the difference?) which generates a random string, and doesn't take any input.
I was able to add a filter like this:
$twig->getEnvironment()->addFilter(
new \Twig_Filter('guid', function(){ return generateGUID(); })
);
But I can't seem to use it without providing some dummy input:
{{ 0|guid }} This will work
{{ guid }} This will not work
How can I use my guid filter/function without providing any input?

Answer 1

A filter always apply on something, it filters something.
What you want is a function, indeed.
The extending Twig page of the documentation is an incredible source of information on that matter.
At first glance, I would even have said you should define a tag for this but the documentation on the tag, explicitly says:
If your tag generates some output, use a function instead.
Source: https://twig.symfony.com/doc/3.x/advanced.html#tags
So indeed, in order to define a function:
Functions are defined in the exact same way as filters, but you need to create an instance of \Twig\TwigFunction:
$twig = new \Twig\Environment($loader);
$function = new \Twig\Twig_Function('function_name', function () {
// ...
});
$twig->addFunction($function);
So more specifically for you:
$container->get('view')->getEnvironment()->addFunction(
new Twig_SimpleFunction('guid', function(){ return generateGUID(); })
);
Will be accessible via:
{{ guid() }}
Other worth reading:
extending twig, in Slim documentation
you can achieve the same with a macro