Re: [Ocaml-lib-devel] Re: Unzip module

[Richard J. <ri...@an...>]

On Sat, Jul 24, 2004 at 12:32:43PM +0200, Nicolas Cannasse wrote:
> As for me, the best format would be OCaml one. For example if you want to
> serialize a  string option list , you will get :
> 
> [None;Some "hello";None;Some "world";None;None;Some "\n"]

I guess there's two sorts of serialisation here.

Using [eg] Yaml, you get interoperability with other languages, which
is a worthwhile practical goal.

A specialised OCaml-specific serialisation would be useful for reasons
of speed.  The Marshal module does this already, albeit in C, to a
changeable binary-only format, with type safety issues.

I've thought hard about whether it would be possible to have a version
of Marshal with these three desirable properties:

(a) Written in pure OCaml.

(b) Uses a sensible text-based format with long-term stability.

(c) Type safety - At a minimum, throws a useful exception, rather than
segfaulting.  Better still if it could "upgrade" data when the type
changes.

So, (a) seems feasible using Obj.  I've examined the source to Marshal
(trying to fix an elusive bug once), and there doesn't seem to be
anything in particular which requires the use of C.  Marshal looks
like it could be rewritten in pure OCaml (and if there are any reasons
why not then Obj should be extended to support it).

(b) is possible, even without hacks like trying to access .cmi files
which might not exist.  For example, any block with three elements can
be written out as:

  (v1, v2, v3)

even though this might represent: a triple; a three-element array of
unboxed types; a three-element struct.  You'd probably want to
special-case things like lists (which would otherwise look like a load
of nested pairs).

Type safety (c) seems quite hard to achieve.  There's not enough
information around at runtime to do this, so you'd need to get the
compiler to write extra information out, which would go counter to the
philosophy of OCaml.

Rich.

-- 
Richard Jones. http://www.annexia.org/ http://www.j-london.com/
Merjis Ltd. http://www.merjis.com/ - improving website return on investment
If I have not seen as far as others, it is because I have been
standing in the footprints of giants.  -- from Usenet