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

[Alain Frisch <alain@frisch.fr>]

On 06/02/2010 03:43 PM, Julien Signoles wrote:
> If I well understood what Alain Frisch and Xavier Leroy explained,
> modules (including both structures and functors) become first class
> value: structures may be converted to records and functors to functions;
> and conversely. But I let more informed person confirm this.

Not really. What you describe is the internal compilation strategy for 
modules (structures -> records; functors -> functions), which did not 
change with the introduction of 1st class modules.

What's possible in OCaml 3.12 is to turn a module (be it a structure or 
a functor) into a "package", which is a first-class value. A package is 
just the wrapper around a module that makes it a first-class value. In 
particular, a package that wraps a structure (resp. a functor) is *not* 
converted to a record (resp. a function).

What you can with a package:
- whatever you can do with a first-class value (pass it to a function, 
store it in a data structure, etc);
- unwrap it back into the underlying module.


We've been using first-class modules for more than one year at LexiFi 
and we have never found a use for packing functors into first-class 
values, but I've no doubt people will find some. In particular, this 
allows the program to decide at runtime (given an environment variable 
or a command-line argument) to use some particular (functorized) 
implementation of a data structure over another.


Alain