File named TAGS generated from hasktags (in case of Haskell) that gives you an easy way to "jump to definition" from Emacs or other editors. Good way to navigate codebases even if you don't know how to build them.

Presumably etags/gtags/hasktags etc., ie. having built a TAGS database for such a helper program, you can use it in an editor to jump from a field name to its definition. That wouldn't be the case with a lens accessor.