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Date: Tue, 9 Jan 96 09:04:10 +0100
From: boos@gr6.u-strasbg.fr (Christian Boos)
Message-Id: <9601090804.AA08645@gr6.u-strasbg.fr>
To: caml-list@pauillac.inria.fr
Cc: arc@labri.u-bordeaux.fr
Subject: Re: modules local to functions.
In-Reply-To: <9601081207.AA14189@waves.labri.u-bordeaux.fr>
References: <9601081207.AA14189@waves.labri.u-bordeaux.fr>
Sender: weis



 > I have a functor such as follows:
 > 
 >   module type Specifyn = sig val n : int end;;
 > 
 >   module Makespecific(N:Specifyn) = struct
 > 	type mytype = int
 > 	let mirror () : mytype = N.n
 >   end;;
 > 
 > I can apply it at the main level like:
 > 
 >   module Fred = Makespecific(struct let n = 7 end);;
 > 
 > However, there are cases when I want to define a module in the middle
 > of a function (eg a functor of packed binary arrays where I want to (...)


Hello,

I have encountered a similar problem yesterday! 
My understanding is that YOU CAN'T do that in CSL at all: functor
application means code generation, and there's no runtime code
generation!

One way to circumvent this problem is to change slightly the semantic
of your argument module, for example:

   module type Specifyn = sig val n : unit -> int end
				      ^^^^^^^

This should be not too inconvenient, and almost as simple to use as
yours:

    module Makespecific(N:Specifyn) = struct
       type mytype = int
       let mirror () : mytype = N.n ()
				    ^^
    end
    
    let toto_fred_n = ref 10
    
    module Fred = Makespecific(struct let n () = !toto_fred_n end) 
    
    let toto digits =
      toto_fred_n := digits;
      Fred.mirror ()


(This particular solution may even be expressed simpler:

    module type Specifyn = sig val n : int ref end
					   ^^^
    
    module Makespecific(N:Specifyn) = struct
       type mytype = int
       let mirror () : mytype = !N.n
				^
    end
    
    module Fred_N = struct let n = ref 10 end
    
    module Fred = Makespecific(Fred_N) 
    
    let toto digits =
      Fred_N.n := digits;
      Fred.mirror ()
	
)

Hope it helps!


Christian