[jawher]

Parser combinators would certainly have been easier for me, the library author but not necessarily so for the end user.

Contrast how it is done now:

  [-R [-H | -L | -P]] [-fi | -n] [-apvX] SRC... DST

With a parser combinator based approach:

  and(
    optional(and('-R', or('-H', '-L', '-P'))),
    optional(or(
                '-fi', 
                '-n'
             )
    ),
    optional('-apvX'),
    repeatable('SRC'),
    'DST')

And this didn't even handle the fact that options order is not important.

[jpdarago]

For that I think you can do something similar to what mpc for C (https://github.com/orangeduck/mpc) and LPeg for Lua (http://www.inf.puc-rio.br/~roberto/lpeg/), which is to provide the parsing machinery and write a small DSL with the same machinery for the users.