If a user provides an invalid option, most applications would want to display the banner.

tools.cli throws an Exception with minimal information, and (banner-for) is marked private.

The exception could be replaced with one that provides access to the banner, but I think it would be better (easier to use, more idiomatic?) to change the contract so that the banner is always returned. Given the exception string is also useful, perhaps change cli to return a map with keys [:options :extra-args :banner :parse-failure]?

As for :parse-fn, a function can be supplied, this
is useful for standard use-cases such as: -vv, or when
you want to build a list from values.

PR: https://github.com/clojure/tools.cli/pull/11

I was trying to use tools.cli in conjunction with a configuration map
loaded from a file, and use clojure.core/merge to combine the results.

This didn't work because tools.cli always uses a default value of
nil, even when no default value is specified. The nil always
overrides defaults from another source.

Example before the patch:

(def my-defaults
  {:foo 1})

(merge my-defaults
       (first (clojure.tools.cli/cli
               [] ; no arguments given
               ["--foo"]))) ; no default specified
;;=> {:foo nil}

This enhancement modifies tools.cli to completely omit arguments which
have no default specified and no value given on the command line.

After the patch, the above example returns:

;;=> {:foo 1}