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From: Xavier Leroy <xleroy@pauillac.inria.fr>
Message-Id: <199609130902.LAA08573@pauillac.inria.fr>
Subject: Re: Caml semantics
To: reichel@tcs.inf.tu-dresden.de (Horst Reichel)
Date: Fri, 13 Sep 1996 11:02:13 +0200 (MET DST)
Cc: caml-list@pauillac.inria.fr
In-Reply-To: <9609121224.AA01164@ithif17.ithi> from "Horst Reichel" at Sep 12, 96 02:24:06 pm
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> What is the formal semantics of recursive data types in Caml and Ocaml?
> 
> Since recursive values like d and e defined by
>    let rec d = 3 :: e  and e = 5 :: d ;;
> are typed as "int list" values the semantics of recursive type definitions
> ist not the initial algebra of least fixpoint semantics (as in SML).

Recursive definitions of data structures (as above, and as opposed to
the usual recursive definitions of functions, let rec f x = ...)
is a Caml Light / Objective Caml extension that is not part of the
core Caml language, nor of SML. If you don't use it, you get the least
fixpoint semantics you mentioned (values of data types are finite
terms).

With "let rec" over data structures, values of data types are rational
terms: infinite terms with a finite number of distinct sub-terms.
Equivalently, these are infinite terms that you can represent as a
finite graph, and that's exactly how they are implemented (with cycles
between the memory blocks that represent each constructor application).

Notice that these rational terms are fully evaluated at the time they
are built. No lazy evaluation of subterms occurs.

> If we take all finite and infinite lists, one would take the
> terminal co--algebra or greatest fixpoint semantics.

Hmph. If it's not the least fixpoint, it must be the greatest
fixpoint? No, there are interesting in-betweens.

> In that case I would
> expect lazy evaluation to get a value also for
>      head(merge(d,e));;
> with
> let rec merge = function ([],l) -> l | (l,[]) -> l
>                        | (x :: l1,l) -> x :: merge(l,l1) ;;
> The present implementations get out of memory during evaluation.

Again, there is no lazy evaluation implicit in recursive data structures.
Programming over these data structures is similar to working with
graphs: one needs to keep track of the nodes already visited, using
e.g. pointer equality (==) or an extra identifier field in each node.

For example, here is a simple-minded function that compares two finite
or infinite lists for equality.

        let list_equal l1 l2 =
          let rec list_equal_aux l1 l2 seen = 
            if already_seen l1 l2 seen
            then true
            else begin
              match (l1, l2) with
                 ([], []) -> true
               | (h1::t1, h2::t2) ->
                   h1 = h2 && list_equal_aux t1 t2 ((l1,l2) :: seen)
               | (_, _) -> false
            end
          and already_seen l1 l2 = function
              [] -> false
            | (a1, a2) :: rem ->
                (l1 == a1 && l2 == a2) || already_seen l1 l2 rem
          in
            list_equal_aux l1 l2 []

> The present implementations seem to make no difference between
> inductive data types and co--inductive data types, which are
> distinguished in CHARITY. In CHARITY  is  co-induction the tool to 
> define functions on values of co-data types.

Good for CHARITY, but again the Caml datatypes are not co-inductive.

> How I can define
> functions in Caml or Ocaml which manipulates recursive types like
> d and e above. A reference to streams is not a satisfying answer, since
> in the more general situation given by 
> 
> type 'a tree = Node of ('a * 'a tree) list ;;
> let rec a = Node([(1,b);(2,c)]) and
>         b = Node([(2,c);(3,Node[])]) and
>         c = Node([(3,a);(4;Node[])]) ;;
> 
> one gets a, b c of type "int tree" and the "stream" module does not help
> any more. Recursive values of type "'a tree" allow the representation
> of infinite computation trees used in CCS. How one can define the
> process composing operations of process algebras in Caml for these
> infinite computation trees?

If your computation trees are rational (i.e. they don't need to be
lazily evaluated), then you can use the data type definition above and
operate over values of these types with graph-like algorithms.

Another option is to do lazy evaluation manually, with explicit
"freeze" and "eval" operations. Something like

        type 'a tree = Node of ('a * 'a tree) delay list

where 'a delay is a type implementing lazily evaluated values of type 'a:

        type 'a delay = { mutable status: 'a status }
        and 'a status = Delayed of (unit -> 'a) | Evaluated of 'a

But be warned that mixing lazy data structures within an otherwise
strict language (call-by-value, side-effects) is fairly tricky. If
what you really need is lazy evaluation, you'd be better off using a
purely functional language with call-by-need semantics, e.g. Haskell.

Regards,

- Xavier Leroy