Re: [Caml-list] type abbreviation is cyclic

["Till Varoquaux" <till.varoquaux@gmail.com>]

Yes,
This used to be a FAQ entry: unless you pass the -rectypes option to
ocaml you have to pass through a constructor (record variant...) to
break your recursion, this prevents writing empty types such as:
type pb = pb * pb
You can still write infinite recursive definitions :
type a = Succ of a;;
let rec infinity = Succ infinity;;

AFAIK -rectypes doesn't break any of the programs working before but
if you attend to use it you will probably run in problems with type
inference (not polymorphic enough). I would recommend steering clear
of it for anything else than toy programs.
Till
On 10/24/07, Brian Hurt <bhurt@janestcapital.com> wrote:
> William W Smith wrote:
>
> > I wonder whether this error is an example of the language being
> > defined more restrictively than required.  What is the reason that I
> > get these results?
> >
> > type a  = int -> one -> int and one = Unused | One of a;;
> > type b = int -> b -> int
> >
> > type a is accepted while type b is not. (b gives "The type
> > abbreviation b is cyclic"  However, in the uses that I intended, there
> > won't be any actual difference between the two.
> >
> > I'd appreciate an explanation about why there is difference  between a
> > and   b.
>
>
> I *think* this has to do with being able to bottom out data structures.
> I mean, consider the following two (similiar) types:
>
> type a = int * one * int and one = Unused | One of a;;
> type b = int * b * int;;
>
> How do you specify a limited-length member of type b?  You can with a
> just by inserting Unused in the correct location.  But there's no such
> "stopping point" for b.
>
> This makes more sense when you make the linked-list aspect more plain:
>
> type a = int * next and next = EmptyList | Cons of a;;
> type b = int * b;;
>
> Hopefully someone more knowledgeable than I will post an actual
> type-theoretical reason this is so, but this is the pragmatic reason
> I've found.
>
> Brian
>
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