How to use modules in place of Type Classes

I'm writing a library to do command line parsing in SML much like
the C getopt() function.  I take a dictionary of expected arguments
and their types and a command line string.  I return a dictionary of
parsed arguments.

I don't really care what sort of dictionary I'm passed.

In order for me to make a generalized GETOPT that accepts any sort of
dictionary, it seems that I have to write my module as a functor, taking
a structure argument and that each of my callers has to explicitly
evaluate the functor.  This means that the implementation information
about the underlying representation of a particular dictionary needs to
be threaded through any code base that uses my library all the way down
to the invocation of getopt().

This seems to add a lot of programmer overhead.  Is there a better way?
Is there any equivalence to the type class sugar of Haskell that I could use?

In practice, do SML users just accept a very specific dictionary and have
the caller do the manual conversions?  Do they functorize everything?

-David

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