I can't stop but see here parallels between cmdline arguments vs. environment variables for programs, and keyword arguments vs. dynamic binding for functions in a program (particularly in a Lisp one).

That is, $ program --foo=bar vs. $ FOO=bar program

seems analogous to:

  (function :foo "bar")
  ;; vs.
  (let ((*foo* "bar"))
    (function))

I suppose the latter argument applies to env vars as well - you can accidentally pass values differing from expectation because you didn't know a parent process changed them. The former doesn't, because processes don't have a "signature" specifying its options, at least not in a machine-readable form. Which is somewhat surprising - pretty much all software written over the past decades follows the pattern of accepting argc, argv[] (and env[]), and yet the format of arguments is entirely hidden inside the actual program. I wonder why there isn't a way to specify accepted arguments as e.g. metadata of the executable file?