I am a Debian developer packaging Clojure and Leiningen. Leiningen2 depends on this project and I'd like to package it. Could you please display copyright holders (developers) for this work? project.clj is likely a good place to note that.

On Java 11 and 12-ea:

 java.lang.ClassNotFoundException: sun.misc.BASE64Decoder, compiling:(flatland/useful/compress.clj:1:1)

java.util.Base64 would be good, but it's only available on Java 8 onwards. Probably still the best choice, but would mean dropping support for Java 7 and down, at least for the flatland.compress namespace.

((ignoring-nils +) nil nil) returns 0, but ((ignoring-nils +) nil nil nil) returns nil. Is this difference intentional? I figure they should both return 0.

Travis CI is only setup on amalloy/useful, not flatland/useful. Could you set it up on flatland/useful as well?

Just a heads up: 0.8.9 and 0.9.0 have not been pushed to clojars (not sure if this is intentional).

It would be great if you could deploy a signature for the latest
release of this library and make it a habit of deploying signed jars
in the future.

Here's a quick description of how to sign already-deployed jars on Clojars:

https://github.com/technomancy/leiningen/wiki/SigningDeployedJars

https://groups.google.com/forum/#!topic/clojure/l8vGw458nFQ contains the details of if-let-all macro.
Is useful an appropriate project to host if-let-all macro?

clojure.core now comes with an update function itself: https://github.com/clojure/clojure/blob/master/changes.md#25-update---like-update-in-for-first-level

This gets replaced by useful.map/update now. Probably the function in useful.map can be removed.

Right now the map-keys function only maps the top keys, i.e:

(map-keys str {:a {:b "c"}})
=> {"a" {:b "c"}}

Can we consider adding something like map-all-keys, map-all-values, etc that will traverse a nested structure? The use case here could be transforming a clojurefied map into a formatted JSON object or vise versa for example.

(defn transform-keys
  "Recursively transforms all map keys in coll with t."
  [t coll]
  (let [f (fn [[k v]] [(t k) v])]
    (clojure.walk/postwalk (fn [x] (if (map? x) (into {} (map f x)) x)) coll)))

(transform-keys ->kebab-case-keyword {"oneTwo" 12 "threeFour" {"fiveSix" 56}})
=> {:three-four {:five-six 56}, :one-two 12}

OR

(defn transform-keys 
  "Recursively transforms all map keys in m with f."
  [f m]
  (letfn [(mapper [[k v]] [(f k) (if (map? v) (map-keys f v) v)])]
    (into {} (map mapper m))))

(transform-keys ->kebab-case-keyword {"oneTwo" 12 "threeFour" {"fiveSix" 56}})
=> {:three-four {:five-six 56}, :one-two 12}

@amalloy Should useful.config also work for non-resources? If so, we could use it for lazybot and get rid of clj-config entirely.