I’ll save here a reddit discussion, which I find interesting but that will be burried quickly down reddit’s history. The goal is to get all the dependencies of a system.

You’d better read the OP’s question and the discussion (where the OP is the experimented svetlyak40wt/40ants, at the moment doing a god’s work on Weblocks).

His solution is https://gist.github.com/svetlyak40wt/03bc68c820bb3e45bc7871870379c42e

(ql:quickload :fset)

(defun get-dependencies (system)
  "Returns a set with all dependencies of a given system.
   System should be loaded first."
  (labels ((normalize (name)
             (etypecase name
               (string (string-downcase name))
               (symbol (normalize (symbol-name name)))
                (let ((dep-type (first name))
                      (supported-dep-types (list :version :feature :require)))
                  (unless (member dep-type
                    (error "This component \"~A\" should have first element from this list: ~A."

                   (case dep-type
                     (:version (second name))
                     (:feature (third name))
                     (:require (second name)))))))))

    (let ((processed (fset:set))
          (queue (fset:set (normalize system))))

      (do ((current-name (fset:arb queue)
                         (fset:arb queue)))
          ((null current-name)
           ;; return result

        ;; Remove current name from the queue
        (setf queue
              (fset:less queue current-name))
        ;; And put it into the "processed" pool
        (setf processed
              (fset:with processed current-name))

        ;; And add it's dependencies which aren't processed or in the queue already
        ;; Sometimes system can't be found because itself depends on some feature,
        ;; for example, you can specify dependency as a list:
        ;; and it will be loaded only on SBCL.
        ;; When we are collecting dependencies on another implementation,
        ;; we don't want to fail with an error because ASDF is unable to find
        ;; such dependencies
        (let* ((system (ignore-errors
                        (asdf:find-system current-name)))
               (deps (when system
                       (asdf:component-sideway-dependencies system))))
          (dolist (dep deps)
            (let ((normalized-dep (normalize dep)))
              (unless (or (fset:lookup processed normalized-dep)
                          (fset:lookup queue normalized-dep))
                (setf queue
                      (fset:with queue normalized-dep)))))))

      (values processed))))

DEPENDENCIES> (ql:quickload :clinch)
DEPENDENCIES> (get-dependencies :clinch)
  "trivial-features" }

There’s also (ql-dist:dependency-tree "mgl") which has limitations though, it’s only for Quicklisp projects and doesn’t work with everything (see the thread).

That’s all folks !