On latest Clojure alpha, we added :keys! (and :syms!/:strs!) check for key presence. They do not check the value (by design). So a map can have the key, with a nil value, and still pass the check:
(let [{:keys! [a]} {:a nil}]
a)
;; => nil
Sometimes nil is not a valid value for a given key. In those cases, a programmer may want to avoid adding the key at all, rather than adding it with nil. This is the most common idiom today:
(cond-> m
(some? v) (assoc k v))
A dedicated assoc-some function would help. It skips the key when the value is nil:
(assoc-some {:a 1} :b 2)
;; => {:a 1 :b 2}
(assoc-some {:a 1} :b false)
;; => {:a 1 :b false}
(assoc-some {:a 1} :b nil :c nil)
;; => {:a 1}
