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From: Leo White <lpw25@cam.ac.uk>
To: Ben Millwood <bmillwood@janestreet.com>
Cc: Jeremy Yallop <yallop@gmail.com>,  David Allsopp <dra-news@metastack.com>,  OCaml List <caml-list@inria.fr>
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Subject: Re: [Caml-list] Match error with abstract types in modules

Ben Millwood <bmillwood@janestreet.com> writes:

> I think the issue would be substantially mitigated were it not for the simple fact that [type 'a t] in a signature
> means "abstract type", whereas [type 'a t] in a structure means "empty type". The fact that there is no syntactic
> distinction, and indeed *no way to make one* caused me a great deal of confusion some time ago when I had a problem
> similar to David's.

You can make an empty type (or at least a type for which there are no
expressions) using:

type 'a t = private T