Re: [Caml-list] Function returning recursive lists

[Julien Blond <julien.blond@gmail.com>]

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Hi Eric,

I tried to do something like that some times ago, while my concern was
about some tricky recursive module definition, i think the problem was the
same : the chicken-egg one. Your docycle is trying to define something it
must already know and the problem can't be escaped for evaluation order
prevents any value to be recursive at definition time. The recursivity can
be achieved by using a second construction pass that corresponds to
additional properties (lazyness or mutability or hacking through Obj).
Those properties will necessarily occur either explicitly (like OCaml) or
implicitly (like in Haskell).

So, the real question, as far as i understand your problem, is : are you
asking how to hide those properties in OCaml or to see if you need to ask
your purchasing department some quantum computer that can predict the
address of a future allocated block ? ;)

-- Julien


2012/12/18 Eric Jaeger <eric.jaeger@ssi.gouv.fr>

> Hi everyone,****
>
> ** **
>
> There are various discussions on recursive lists in the archive, yet I was
> wondering whether or not it was possible in pure OCaml to write a function
> returning non-constant recursive lists.****
>
> ** **
>
> For example, I would like to have a function “docycle:’a list->’a list”
> that takes a non recursive list and transforms it into a recursive list
> containing the same elements. That is, “docycle [1;2;3]” would return a
> list structurally equivalent to “let rec c=1::2::3::c in c”. So far, my
> various attempts (OCaml 3.12) have not been successful. Another good
> example is to have a List.map compatible with recursive lists.****
>
> ** **
>
> Please note that it is, in a way, a theoretical (and possibly naïve)
> question :****
>
> **-          **I do not consider recursive lists as the perfect
> implementation for my problem****
>
> **-          **I do not care about efficiency****
>
> **-          **I do not want to use an ad hoc mutable/lazy list datatype
> (unless I’ve also a conversion function toward standard lists)****
>
> **-          **I do not want to use Obj or other similar tricks****
>
> It’s just that I’m curious whether or not what I’m trying to achieve is
> possible.****
>
> ** **
>
>   Regards, Eric****
>
> ** **
>

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