I don't know how to make the `command-line` syntax do this (there's no
flag-clause like `#:required-once`). So I think I'd just do a normal
test outside it:
(define destination (make-parameter #f)) ;default to #f meaning "unspecified"
(command-line
.... your existing code ....)
(define (main)
(unless (destination)
(raise-user-error "The -d <destination> option is required."))
.... your existing code ....)
p.s. Not too many versions ago Racket added a `main` submodule [1].
Using that would let other, "helper" functions be used and tested
independently. However for what you have, now, the way you're doing
it also seems fine.
[1]:
http://docs.racket-lang.org/guide/Module_Syntax.html#%28part._main-and-test%29
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