Snippet that registers a Datomic attribute with function valueType; then create an enitity with a function. Next, retrieve the function via a query, bind it to a var, and invoke it using params.

identless-datomic-functions.clj

(require '[datomic.api :as d])

(def uri "datomic:mem://hello")
(d/create-database uri)


;; Define a transaction to create an attribute with type function (:some/fn)
(def attribute-tx [ {:db/id (d/tempid :db.part/db)
                     :db/ident :regular/attribute
                     :db/cardinality :db.cardinality/one
                     :db/valueType :db.type/fn
                     :db.install/_attribute :db.part/db} ])

;; Define a transaction to create a function
(def fn-tx  [{:db/id (d/tempid :db.part/user)
              :regular/attribute #db/fn {:lang :clojure
                              :params [name]
                              :code (str "Hello, " name)}}])

;; Transact
(d/transact (d/connect uri) attribute-tx)
(d/transact (d/connect uri) fn-tx)

;; Query and bind result to var
(def hello (ffirst (d/q '[:find ?v
                          :where
                          [?e :regular/attribute ?v]]
                     (d/db (d/connect uri)))))

;; Invoke function with params
(hello "Jordan!")
;; => "Hello, Jordan!"