[m-rev.] deduplicating getopt modules

[Peter Wang]

On Sat, 19 Sep 2020 13:09:44 +1000 "Zoltan Somogyi" <zoltan.somogyi at runbox.com> wrote:
> 
> 2020-09-19 12:46 GMT+10:00 "Peter Wang" <novalazy at gmail.com>:
> >> Or are you proposing that process_options take a pair of arguments
> >> of type T, which *may* be I/O states at some call sites but not others, and that
> >> the type class constraint be on T? 
> > 
> > Something like:
> > 
> >     :- typeclass file_special_handler(T, S) <= (S -> T) where [ ... ].
> > 
> >     :- instance file_special_handler(io.input_stream, io.state).
> > 
> >     :- type no_file_special_handle.
> >     :- type no_file_special.
> >     :- instance file_special_handler(no_file_special_handle, no_file_special).
> > 
> >     :- pred process_options(option_ops(OptionType)::in(option_ops),
> > 	list(string)::in, list(string)::out,
> > 	maybe_option_table(OptionType)::out, S::di, S::uo) is det
> > 	<= file_handler(S, T).
> 
> I don't understand what the T argument of the typeclass is intended to be for.
> The only operation the typeclass needs is "given a filename, return either
> a string that is the contents of the file it names, or an error description",
> which does not need any type beyond the I/O state.

You're right. I was thinking T, the file handle, was required because
the typeclass would have separate methods to open the file, read from
the file handle, and close the file handle.

Peter