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Message-ID: <4863F411.4030701@ed.ac.uk>
Date: Thu, 26 Jun 2008 20:54:57 +0100
From: Jeremy Yallop <jeremy.yallop@ed.ac.uk>
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To: caml users <caml-list@inria.fr>
Subject: Recursive modules and variance
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X-Spam: no; 0.00; recursive:01 annotations:01 recursive:01 ocaml:01 sig:01 struct:01 sig:01 struct:01 rec:01 modules:02 modules:02 parameter:02 parameter:02 covariant:02 module:03 

Should variance annotations be visible to modules in the same recursive 
group?

The following program is accepted by OCaml 3.10.2

        module M : sig type +'a t end =
                struct type +'a t end
        module N : sig type +'a t = 'a M.t end =
                struct type +'a t = 'a M.t end

but the following program is rejected

    module rec M : sig type +'a t end =
                struct type +'a t end
           and N : sig type +'a t = 'a M.t end =
                struct type +'a t = 'a M.t end

with the message

    "In this definition, expected parameter variances are not satisfied.
     The 1st type parameter was expected to be covariant,
     but it is invariant"

Jeremy.