You can't use map or vector binding in the params of a datomic function map. It will cause a 'clojure.lang.PersistentVector cannot be cast to java.lang.Stringclojure.lang.PersistentVector cannot be cast to java.lang.String

datomic-function-binding.clj

;; You can't use map or vector binding in the params of a datomic function map.  It will cause a 
;; 'clojure.lang.PersistentVector cannot be cast to java.lang.Stringclojure.lang.PersistentVector cannot be cast to java.lang.String

;; For example, I was attempting ot create a function so that I could pass the following to the transactor function:
;; (d/transact *conn* [ [:node/add {:node/description "Hello" :node/uri "http://www.example.com" :node/tags ["tag1", "tag2"]]])
;; This causes an error, because params isn't being treated like it can be bound to.  You can only specify symbols to bind to
(def add-node {:db/doc "A function for adding a node to the database"
               :db/ident :node/add
               :db/fn (d/function '{:lang :clojure
                                    :params  [db [{:keys [uri description tags prerequisites]}]]
                                    :code [((when-not (seq (d/q '[:find ?e
                                                                   :in $ ?uri ?desc
                                                                   :where
                                                                   [?e :node/description ?desc]
                                                                   [?e :node/uri ?uri]]
                                                                 db uri description))
                                               (let [node-map (hash-map :db/id (d/tempid :db.part/user)
                                                                        :node/uri uri
                                                                        :node/description description
                                                                        :node/tag tags)]
                                                 (if prerequisites
                                                   (assoc node-map :node/prerequisite prerequisites)
                                                   node-map))))]})}

;; The solution is to change params and add a let within the code section to do the binding for you
(def add-node {:db/doc "A function for adding a node to the database"
               :db/ident :node/add
               :db/fn (d/function '{:lang :clojure
                                    :params  [db param-map]]
                                    :code [(let [{:keys [uri description tags prerequisites]} param-map]
                                             (when-not (seq (d/q '[:find ?e
                                                                   :in $ ?uri ?desc
                                                                   :where
                                                                   [?e :node/description ?desc]
                                                                   [?e :node/uri ?uri]]
                                                                 db uri description))
                                               (let [node-map (hash-map :db/id (d/tempid :db.part/user)
                                                                        :node/uri uri
                                                                        :node/description description
                                                                        :node/tag tags)]
                                                 (if prerequisites
                                                   (assoc node-map :node/prerequisite prerequisites)
                                                   node-map))))]})})