Re: [Caml-list] Questions concerning modules as first-class values

[Jacques Garrigue <garrigue@math.nagoya-u.ac.jp>]

From: Alain Frisch <alain@frisch.fr>
> On 02/06/2010 19:36, Eric Cooper wrote:
>> Is it possible to marshal and unmarshal these packages?
> 
> Yes, this is possible, but:
> 
> 1. Extremely dangerous: there is no runtime type-checking. If you
> marshal a module, you need to unmarshal it to the same package type,
> exactly (no coercion allowed).
> 
> 2. Rather useless: the code for functions within modules is not
> marshaled (as for regular functions). If you marshal a module that
> define functions, you need to unmarshal it in the same program.
> 
>> That might be a nice way to implement a plug-in architecture.
> 
> Given the points above, I don't think so!

While marshalling of first-class modules is indeed not a good idea, I
think that dynamic loading of first-class modules might provide a nice
plug-in architecture.
For instance, you can easily build a list of such plug-ins, with an
uniform interface, and add elements to this list through dynamic
loading. They can even expose internal data, with an abstract type.

There is however a difficulty if you want to save and load data used
by these plugins uniformly. Namely, even if all these plugins expose
some data with a type "t", the "t" is actually going to be different
in each, and the type system will enforce it. This is good, but it
will be seen as different for the same plugin in a different run of
the same application. So you need to do some standard hacking around,
having unique ids for each plugin, and some magic to recover the data.
Otherwise this seems a nice approach, easier than using objects, since
objects don't let you expose internal data of different types.
(Well, actually you can probably do this with objects too, using a
polymorphic method to encoding existential types in CPS style, but
this is going to be costly and not very natural.)

Jacques Garrigue