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module types and constraints in several layers
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Date: -- (:)
From: Christian Sternagel <christian.sternagel@u...>
Subject: Re: [Caml-list] module types and constraints in several layers
On Thu, Mar 15, 2007 at 08:25:08AM -0500, Chris King wrote:
> On 3/15/07, Christian Sternagel <christian.sternagel@uibk.ac.at> wrote:
> >but I was not able to tell the compiler (correctly) that after
> >building module E ist should be the case that for example
> >E.E1 = D.D1 = B.B1 = A = E4.D.D1 = E4.D.D2.B1 = ...
> >
> >Is there a convenient way to make such a structure of modules?
> >And if yes, how ist it done?
> 
> What's happening is that, when you specify the type of the results of
> B.Make and D.Make, O'Caml throws away the info about how they were
> created: since the type signatures B.T and D.T simply say "these
> modules have some submodules" but don't say where the submodules come
> from, that info becomes hidden (as it should be).  However you can
> explicitly instruct the compiler not to hide this info:
> 
> module Make (A : A.T) : B.T with module B1 = A = struct
> ...
> module B1 = A
> ...
> end
> 
> This just says "give the resulting module the signature B.T, but
> retain the info that module B1 equals A".  Similarly:
> 
> module Make (B : B.T) (C : C.T) : D.T
>    with module D1 = B.B1 and module D2 = B
>    = struct
In fact thats exactly what I did, but it only worked when a had not more than 1 layer of functors applied to functors. I may need to give my actual program structure in more detail in order to explain my problem better.

variable.mli:
module Make (Id : IDENT.T) : VARIABLE.T with module Id = Id

function.mli:
module Make (Id : IDENT.T) (Lab : LABEL.T) : FUNCTION.T
 with module Id = Id
 and module Lab = Lab

term.mli:
module Make (Var : VARIABLE.T) (Fun : FUNCTION_SYMBOL.T) : TERM.T
 with module Var = Var
 and module Lab = Fun.Lab
 and module Fun = Fun

TERM.mli:
module type T = sig
 type t
 module Fun
 val f1 : t -> t -> bool
 val f2 : t -> t
 val f3 : Fun.t Setx.set -> t -> t
end

rule.mli:
module Make (Term : TERM.T) : RULE.T
 with module Var = Term.Var
 and module Lab = Term.Lab
 and module Fun = Term.Fun
 and module Term = Term

trs.mli:
module Make (Rule : RULE.T) : TRS.T
 with module Var = Tule.Term.Var
 and module Lab = Rule.Term.Lab
 and module Fun = Rule.Term.Fun
 and module Term = Rule.Term
 and module Rule = Rule

TRS.mli:
module type T = sig
 type t
 module Fun : FUNCTION.T
 val f0 : t -> Fun.t Setx.set 
end

everythin compiles fine. Then I make the following in another file.

some.ml:
module Make (Trs : TRS.T) : SOME.T = struct
 module Var = Trs.Var
 module Lab = Trs.Lab
 module Fun = Trs.Fun
 module Term = Trs.Term
 module Rule = Trs.Rule
 module Trs = Trs
 ...
  let ds = Trs.f0 trs in
  Term.f1 (Term.f2 (Term.f3 d s)) t
                            ^
 ...
end

end try to compile an the resulting error message is:

This expression has type Trs.Fun.t Setx.set but is here used with type
Term.Fun.t Setx.set
Type Trs.Fun.t = Trs.Fun.t is not compatible with type
Term.Fun.t = Trs.Term.Fun.t 

where the ^ markes the ment expression

cheers

christian

> ...
> module D1 = B.B1
> module D2 = B
> module D3 = C
> ...
> end
> 
> You don't need to do this with E.Make since its signature is left intact.
> 
> You can do this with individual types too, see section 6.10 of the manual.
> 
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