Print warnings to STDERR to avoid polluting STDOUT

[dmatysiak]

Printing warnings to STDOUT pollutes the output of any program that uses STDOUT. This patch makes sure warnings are directed to STDERR.

[mebble]

Could we also have the ability to suppress warnings? I have an app that logs to stderr and I don't want environ polluting those logs. I was thinking an API that looks something like this:

(env :env-key {:warnings false})

I'm new to clojure and don't know how to work with defonce so I'm unable to raise my own PR

[dmatysiak]

IMO, ideally, we shouldn't be logging this way in the first place (or using environment variables to enable or disable warnings, for that matter). If warnings are a must, a compromise might be to add a dynamic var to enable warnings (and one to override the file descriptor):

(def ^:dynamic *warnings?* false)
(def ^:dynamic *warnings-out* *err*)

(defn- printerr [& more]
  (when *warnings?*
      (apply println more)))

after which you can optionally enable these messages, e.g.:

(binding [environ/*warnings* true]
    ...your naughty code...)