core.clj

(ns typed-clj-exp.core
  (:require [clojure.core.typed :refer [ann check-ns typed-deps
                                        def-alias ann-datatype for>
                                        fn> ann-form AnyInteger doseq>
                                        cf inst letfn> override-method
                                        dotimes> loop>]
             :as t]))

;; avoid  null pointer exceptions at compile time

;; doesn't typecheck because n could be nil
(ann badincfirst [(t/Seq Number) -> Number])
#_(defn badincfirst [sq]
  (let [n (first sq)]
    (inc n)))

;; typechecks
(ann incfirst [(t/Seq Number) -> Number])
(defn incfirst [sq]
  (let [n (first sq)]
    (if n
      (inc n)
      1)))

;; typechecks
(+ 123 (incfirst '(1 2 3)))

;; doesn't
;; (+ 123 (incfirst '(:a)))


;; (check-ns)