Resolving a keyword into a Malli schema from the default registry in Clojure

Question

How do I resolve a keyword to a schema from the default Malli registry? I seem unable to look up a value in the registry in order to walk it.

(def registry
  (atom {}))

(defn register! [type ?schema]
  (swap! registry assoc type ?schema))

;; Combine the default registry with our own mutable registry.
(mreg/set-default-registry!
 (mreg/composite-registry
    (mreg/fast-registry (malli/default-schemas))
    (mreg/mutable-registry registry)))

(register! :db/kasse
   [:map
    [:id                            [:int {:primary-key true :db-generated true}]]
    [:odlingsplats                  [:string {:foreign-key "odlingsplatser"}]]
    [:diameter_m                    :int]
    [:djup_m                        :int]
    [:volym_m2                      [:int {:db-generated true}]]])

(malli/walk
 :db/kasse
 (malli/schema-walker identity))
;; => :db/kasse

I've tried wrapping :db/kasse in different functions from malli but none seem to do the lookup and malli/-lookup is private. Just running (:db/kasse malli/default-registry) does not work either. Using malli/schema seems like the obvious choice but it seemingly has no effect.

(malli/walk
 (malli/schema :db/kasse)
 (malli/schema-walker identity))
;; => :db/kasse

score 0 · Accepted Answer · answered Jan 20 '22 at 16:24

Calling malli/deref was the answer:

(malli/walk
 (malli/deref :db/kasse)
 (malli/schema-walker identity))
;; => [:map [:id [:int {:primary-key true, :db-generated true}]] [:odlingsplats [:postgres/string {:foreign-key "odli\
ngsplatser"}]] [:diameter_m :int] [:djup_m :int] [:volym_m2 [:int {:db-generated true}]] [:namn {:optional true} [:po\
stgres/string {:db-generated true}]]]

Thank you to ikitommi at the Clojurians slack for providing the answer. He also provided an explanation as to why the library works this way:

The :db/kasse returned is a Malli Schema instance, it’s print output is just the form, so looks like keyword. It’s type is :malli.core/schema, which is the internal eager reference, like a Var in Clojure. If you want to get the schema behind it, you can m/deref it. But, calling m/validate on :db/kasse works too. the :malli.core/schema forwards the calls to the actual instance, like Var.

Resolving a keyword into a Malli schema from the default registry in Clojure

1 Answers1