Conditional display of the_excerpt or the_content by category id

I’m not much of a coder, so am struggling to create a function (in functions.php) that will allow me to display the excerpts for posts in one category and the full post for another category. Here’s what I have so far. The excerpts for category 8 displays, but other categories display the words “the_content”.

// show full post or excerpt on categories
function post_length_choice($content) {
//    if (is_category( array(1, 22, 23))) {
     if (is_category(8)) {
         $content= 'the_excerpt';
     } else if  (is_category()) {
         $content= 'the_content';
     }
     return $content;
}
add_filter('the_content', 'post_length_choice');