This, if used without a corresponding filter run prefix (e.g. +) will output all tiddlers in a wiki, since those are the input to a filter run without a constructor as initial operator.
Try this in a new tiddler on tiddlywiki.com to see what I mean:
\function .noop() [all[]]
<$list filter="[[HelloThere]] [.noop[This is a comment]]" />
@CodaCoder got it right. I’m not sure that you can put the necessary logic into a single function definition. I do like the idea of a :noop filter run prefix, though, since this way we don’t have to make the constructor/modifier distinction.