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[Caml-list] Legality of using module types from .mli in .ml
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Date: -- (:)
From: Thorsten Ohl <ohl@p...>
Subject: [Caml-list] Legality of using module types from .mli in .ml
I'm often referring to module types defined in an interface file in
order to reduce redundancy.  For example

    $ cat a.mli
    module type T = sig val n : int end
    module M : T

    $ cat a.ml
    module type T = A.T
    module M : T = struct let n = 42 end

where T can be rather long.  Is this legal?  The manual says in
section 6.12

    A compilation unit behaves roughly as the module definition
    module unit-name : sig  unit-interface end =  struct unit-implementation end 

and refrains from defining `roughly' :-).  I came up with conflicting
evidence:

O'Caml accepts it
    
    $ ocamlopt a.mli a.ml 

However, it is not equivalent to

    module B :
      sig
	module type T = sig val n : int end
      end =
      struct
	module type T = B.T
	module M = struct let n = 42 end
      end
    
which is obviously rejected by O'Caml, since B.T is not bound.  The
pair (a.mli, a.ml) more closely resembles

    module Aux_C =
      struct
	module type T = sig val n : int end
      end
    
    module type C =
      sig
	module type T = Aux_C.T
	module M : T
      end
    
    module C : C =
      struct
	module type T = Aux_C.T
	module M = struct let n = 42 end
      end

without binding Aux.T.  My question is now: is the pair (a.mli, a.ml)
intentionally legal or do I need to expand the reference A.T, as in

    $ cat pedantic_a.mli
    module type T = sig val n : int end
    module M : T

    $ cat pedantic_a.ml
    module type T = sig val n : int end
    module M : T = struct let n = 42 end

because I've have taken advantage of a bug in the compiler.

The reason I'm asking this, is that my shorthand [as in (a.mli, aml)]
breaks Julien Signoles' defunctorizer.

Cheers,
-Thorsten
-- 
Thorsten Ohl, Physics Dept., Wuerzburg Univ. -- ohl@physik.uni-wuerzburg.de
http://theorie.physik.uni-wuerzburg.de/~ohl/     [<=== PGP public key here]
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