TypeScript + Clojure

demo.cljts

;; TypeScript + Clojure

(deftype User {
  :id            Num
  :name          Str
  Optional<:age> Num         ;; Key may be missing, but if present a Num value is present
  :address       <Str | Nil> ;; Key is present, but value can be Nil
  :orders        Seq<Order>  ;; Seq is super type of Vec/ArrayList/etc
})

;; Supports Intersections
(deftype ExtendedUser <User & {
  :anything      Any             ;; Any type is special, like TS
  :metadata      {:some-num Num} ;; Can describe nested maps
  Keyword        Str             ;; This is special, it will accept any other properties so long as the key is a Keyword and value a String
}>)

(deftype Error {
  :status  <:success | :failure> ;; Supports union types
  :message Str
})

;; Defining function types
(deftype CallbackFn [<Error | Nil>] => Any)
(deftype TwoArgCallbackFn [User <Error | Nil>] => Boolean)

;; Type inferred as Long
(def some-const 1)

;; Using explicit type
(def cb: CallbackFn #(println %))
(def cb2: CallbackFn 
  (fn [e] ;; Inferred as <Error | Nil> 
    (println e)))

;; When not using the explicit type, Inferred as ([Any] => Any)
(def cb3 #(println %))    

(def default-error: Error 
  {:status  :failure
   :message "An unknown error occurred"})

;; Note, omitting the type defaults the return to Any and parameters to Any types
(defn change-user-name:<Error | User>
  [user: User name: String]
  (cond 
    (str/blank? name)
    {:status  :failure 
     :message "Name is blank"}

    (db/name-taken? (:id user)) 
    {:status  :failure
     :message "Name already taken"}

    :else 
    (if (db/update-name! (:id user) name)
      (assoc user :name name)
      default-error)))

;; Sample core functions

(defn <T K V>assoc:<T & {K V}>
  [map: <T | Nil> key: K val: V]
  (clojure.lang.RT/assoc map key val)) 

;; keyof special syntax, T[K] is the value type in map
(defn <T <K extends keyof T> N>get:T[K]
  ([map: <T | Nil> key: K]
    (clojure.lang.RT/get map key))
  ([map: <T | Nil> key: K not-found: N]
    (clojure.lang.RT/get map key not-found)))