How to use function "source"?

[crazydeath137]

as I see, Twig has a good function named "source" : https://twig.symfony.com/doc/1.x/functions/source.html
But I can't use it in gulp-twig.
{{ source(file_path) }} --> all I see is a message like this "Template "file_path" is not defined."

this function is very usefull to me, Tks for ur reading.

[crazydeath137]

my solution to include raw file is create a new function.

`

functions:[
{
name: "include_raw",
func: function (path) {
var content = 'content';
content = fs.readFileSync('app/'+path).toString();
return content;
}
}
]

`

and use it
`

{{ include_raw('haha.html') }}

`