code.clj

;;; Changes:
;;; - Annotate the parameter of the lambda given to filter. This was the cause of the error.
;;; - Change the last parameter of lookup-by from Seq to Seqable to allow it to work with vectors.
;;; - Replace (IFn [a -> b]) with [a -> b]. They're equivalent.

(t/ann lookup-by (t/All [a b]
                   [b [a -> b] (t/Option (t/Seqable a)) -> (t/Option a)]))

(defn lookup-by
  "Convenience filter. Returns the first item in coll where (= value (lookup-fn item))"
  [value lookup-fn coll]
  (first (filter (t/fn _ [x :- a]
                  (= value (lookup-fn x)))
                 coll)))

; Example
(lookup-by "Kris" :name [{:name "foo"} {:name "Kris"} {:name "bar"}])

👍 Thanks dude!