[Caml-list] limits on mutual recursion and modules?

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Brian Rogoff

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Nov 22, 2001, 10:55:40 AM11/22/01
to William Harold Newman, caml...@inria.fr
On Wed, 21 Nov 2001, William Harold Newman wrote:
> I'm trying to understand the limits on mutual recursion in ML.
>
> I've seen the hack
> type 'a combination = T1 of int | T2 of 'a | T3 of 'a * 'a
> class virtual test =
> object
> method virtual get: test combination
> end
> for mutual recursion between classes and modules in OCaml. That doesn't

You mean between classes and types here, of course.

> leave me with much confidence that I can figure out whether a particular
> kind of mutual recursion is possible.:-|
>
> I've seen various statements about recursion between modules being
> impossible, but I'm not sure exactly how severe a limitation this is
> in practice, especially given the possibility of hacks like the one
> above.

That hack, which I've seen called the "parameterization trick" (we really
need a better, sexier sounding name for it) is the way you currently
create a recursion between a type definition and a functor instantiation.

> In particular, I'm curious whether it's possible to define
> a record type Foo which contains a functor-defined data structure which
> refer to objects of type Foo. E.g., in OCaml is there any way
> to define a record type Foo one of whose fields is a Set of Foo?

You use that same trick. It also means that you must make a polymorphic
version of Set to participate in the recursion; the library Set won't do.
Check this out

http://caml.inria.fr/archives/200010/msg00154.html

It's my guess that every frequent user of OCaml or SML bangs into this
within their first 9 months of serious ML programming, and most likely
long before that.

> In general, I'd be interested in any pointers to treatments of this
> problem and the theoretical limits involved.

http://cristal.inria.fr/~hirschow/index.html

as he is the one working on it and he has pointers to related work there.

-- Brian
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