Failing Recursive Malli Schema

failing_mutual_recursion.clj

(ns failing-mutual-recursion
  (:require [malli.core :as m]
            [malli.util :as mu]))

;; Define some AST node schemas
;; Includes references to the generic AST schema.

(def Lit
  [:cat [:= :lit] :any])

(def Var
  :symbol)
  
(def App
  [:cat
   [:= :apply]
   [:ref ::AST]
   [:* [:ref ::AST]]])

;; Define the generic AST schema as a union of all possible node types.

(def AST
  [:or
   [:ref ::Lit]
   [:ref ::Var]
   [:ref ::App]])
   
;; Create a custom registery with the new AST schemas 

(def registry
  (merge (m/default-schemas)
         (mu/schemas)
         {::Lit Lit
          ::Var Var
          ::Abs App
          ::AST AST}))

;; Try it out!

(m/validate ::Lit [:lit 1] {:registry registry})
;; => true
;; So far, so good.

(m/validate ::App
            [:apply 'inc [:lit 1]]
            {:registry registry})
;; clojure.lang.ExceptionInfo: :malli.core/potentially-recursive-seqex 
;;  [:ref :erp12.schema-inference.meta-schemas/AST] 
;;  {:type :malli.core/potentially-recursive-seqex, 
;;   :message :malli.core/potentially-recursive-seqex, 
;;   :data [:ref :erp12.schema-inference.meta-schemas/AST]}


;; The `App` schema _is_ mutually recursive with `AST` but all recursive references have
;; been wrapped in a `[:ref _]`. Why is the `App` schemas not usable?