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From: xorque <xorquewasp@googlemail.com>
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Subject: Re: Operation can be dispatching in only one type
Date: Fri, 13 Nov 2009 12:43:42 -0800 (PST)
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Thanks for the response.

It may be that I don't want to design the interface like the above at
all.

The only real requirement is that:

Archivers use a common protocol: Code just sees operations
on Archiver_t'Class and doesn't know if it's reading from a Zip, RAR,
directory, etc.

Presumably, then, when I open a file using an operation from an
archiver, I can't know about the implementation of the actual file
as it's private to the archiver.

Hence, I've ended up with two private tagged types. Is there a better
way (I've not ignored your other suggestions, I'm just exploring other
possibilities first)?