(s/def ::fact
  (s/cat :var (s/? ::?symbol)
         :type (s/or :simple symbol? :acc ::acc-vec)
         :destruct (s/? vector?)
         :exprs (s/* list?)))

(s/def ::fact-without-var
  (s/cat :type (s/or :simple symbol? :acc ::acc-vec)
         :destruct (s/? vector?)
         :exprs (s/* list?)))

There are always macros of course but I was hoping for something easier.

doesn't the first one include the second?

But I want a spec for data where a var is forbidden. And one where it's optional.

ah

I'm not sure I understand what you meant.

so, you can create a new spec that is an s/cat that just contains the common portions, then reference it in both ::fact specs

(s/def ::fact-without-var
  (s/cat :type (s/or :simple symbol? :acc ::acc-vec)
         :destruct (s/? vector?)
         :exprs (s/* list?)))

(s/def ::fact
  (s/cat :var (s/? ::?symbol)
         :tail ::fact-without-var))

that does introduce nesting in the conformed data so is not everything you want

Yeah 🙂

another way would be to restrict the bigger one

maybe something like

(s/def ::fact
  (s/cat :var (s/? ::?symbol)
         :type (s/or :simple symbol? :acc ::acc-vec)
         :destruct (s/? vector?)
         :exprs (s/* list?)))

(s/def ::fact-without-var
  (s/& ::fact #(nil? (:var %))))

can't say I've done this before, might need to tweak that predicate more

Hm, gotta read up on &. Thanks!

& just lets you and arbitrary predicates into the regex spec

Ah, I see. Could work. Thanks!