Twig not detecting the custom function - php

Question

I have written a Twig custom function (in a Twig custom extension). I notice the template does not read the function and keeps throwing me the "Method Exists" error.
Was wondering if you have faced this before. Any ideas ?
The custom extension file; $post and $list are both objects:
<?php
namespace App\Twig;
use Twig\Extension\AbstractExtension;
use Twig\TwigFunction;
class ShowContentExtension extends AbstractExtension
{
public function getFunctions()
{
return [
new TwigFunction('ShowList', [$this, 'ShowList']),
new TwigFunction('ShowPost', [$this, 'ShowPost'])
];
}
public function ShowList($list) {
foreach ($list->post as $post) {
$this->ShowPost($post);
}
}
public function ShowPost($post) {
return !((count(array_keys($post->documents))) < 2 && array_key_exists('header', $post->documents));
}
}
and this is where they are being called:
{% if ShowList(list) %}
<h2 >{{ title }}</h2>
<div>
<div>
{% for post in list %}
{% if post|length > 0 %}
{% include ./_links.html.twig' with { 'list': list} %}
{% endif %}
{% endfor %}
</div>
</div>
{% endif %}
and :
{% if ShowPost(post) %}
<div>
<div>
<a href="{{ post.link }}">
<span>{{ post.title }}</span>
</a>
</div>
</div>
{% endif %}
and this is the screenshot of the error :
error screenshot