On Sun, 2004-05-02 at 05:51, Alain.Frisch@ens.fr wrote:
> On 2 May 2004, skaller wrote:
> 
> > It can now do slightly better than that. It is possible to use
> > the new 'private' keyword to *guard* your mutable list.
> >
> > module MList = struct
> > type 'a mylist = private { head : 'a; mutable tail : 'a mylist option; }
> > ..
> > let splice a b = ...(* makes a new mylist of a @ b *)
> > end
> > Here, lists are immutable *publically*.
> 
> Not quite.
> 
> First, the "private" keyword should be used only in the signature, not the
> structure, otherwise the implementation of the module has no special
> right. Something like:
> 
> module M :
>  sig type t = private *** end
> =
>  struct type t = *** end

Thanks for the correction. Also note I chose a *really bad name*
when I called it 'splice': the intent was to construct
a new list as stated, so I should have just called it 'concat'.

Of course you could make a list with an actual splice mutator,
but then it wouldn't be immutable.

> Second, the client cannot create values of the "private" type.  This is
> not related at all with the mutability of the fields. It can only
> deconstruct values (with pattern matching). 

Right. Well stated.

Hmm ..

 module M : sig
  ?? type 'a t = private 'a list
    val uniq_cons: 'a t -> 'a -> 'a list
    val empty: unit -> 'a list
  end = struct
    type 'a t = 'a list
    let uniq_cons lst a = if List.mem a lst then lst else a::lst
    let empty () = []
  end
  ;;              ;;
Syntax error: 'end' expected, the highlighted 'sig' might be unmatched

Arggg.. what's wrong here? It has nothing to do with
either private or type variables, and the above sig
fails immediately in a module type statement too.

-- 
John Skaller, mailto:skaller@users.sf.net
voice: 061-2-9660-0850, 
snail: PO BOX 401 Glebe NSW 2037 Australia
Checkout the Felix programming language http://felix.sf.net



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