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

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). So you have to expose
constructors explicitly (splice is one of them, but you probably want a
simple Cons).


-- Alain

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