Module hierarchy revisited

From: John Prevost (
Date: Sat Dec 04 1999 - 07:45:44 MET

Subject: Module hierarchy revisited
From: John Prevost <>
Date: 04 Dec 1999 01:45:44 -0500

Idea for namespace management, followed by a question or two, and
explanation of why.

I just came up with what seems like a reasonable way to package my
modules hierarchically (to avoid namespace collisions) in a reasonable

The idea:

For each "package" of stuff, the various modules (individual object
files) have short names, like "Foo", "Bar", and "Baz". The danger, of
course, is that other packages from other sources will have names like
Foo Bar and Baz, because they're so short.

My current working solution is to add as the last object file
something like this:
module Foo = Foo
module Bar = Bar
module Baz = Baz

Why am I doing this? Well, with Gord Stolpmann's findlib, there's a
convenient way to link against a bunch of "packages" by name. This is
nice, because you don't have to manage ordering and -Iing of all of
those files and directories by hand. The difficulty is that control
over which modules come first is a problem. Say we have two packages:

package1 => package1.cma (foo.cmo frubble.cmo)
package2 => package2.cma (foo.cmo bar.cmo zotz.cmo)

If I do:

ocamlc package1.cma package2.cma

I lose access to package1's Foo module in any files I use after this.
If I do it in the other order, I lose access to package2's Foo module.
I can work around this by putting things that want foo from
package1.cma before the reference to package2.cma, but this can be a
pain. I could also insert a file between the two which binds
package1's Foo to something else:
module Zot = Foo

Now I can refer to Foo from either package later on (which is
important if I want to use both in one file, or I have dependencies
which don't allow a simple ordering of my modules and the libraries.)
Even more, it's a pain to have to think about this stuff. I'd rather
just be able to know it won't happen.

What adding the extra module at the end of the library does for me as
a library author is arrange for an "automatic" binding like this to
take place. Mypackage.Foo Mypackage.Bar and Mypackage.Baz will always
be the modules from Mypackage unless Mypackage is shadowed. And the
namespace of packages tends to be nicer and cleaner than the namespace
of individual modules in those packages. (Say, Text.Parser for
low-level Unicode parsers vs XML.Parser for a module that does XML
parsing.) One could extend this further by having super-packages
which provide namespace to a number of other packages:

module Apollo =
    module XML =
        module Parser = ...
    module Text =
        module Parser = ...


module Apollo =
    module XML = XML
    module Text = Text

So the question I have is whether people think that organizing things
in this manner is a Good Thing, and if people have opinions on whether
there's a Right Way to go about doing this and choosing names for
things. As an example, I have a package I call "text" which has a
text.cma containing a module Text which points at the other modules by
name. But the name "Text" is pretty broad, and could collide easily
with other people, even when both packages would be useful.

A second question is whether anyone has recommendations for hiding the
"other" bindings of modules (i.e. I don't want Iso_10646 to appear in
the top-level namespace, I only want Text to appear, containing
Text.Iso_10646) to keep people from referring to the modules in less
safe ways.

I'm thinking about this because I'd like to put some modules out there
for people to use, and the community-driven standards in the world of
Perl, for example, allow huge numbers of modules from all over to be
mixed and matched at will. O'Caml stuff, on the other hand, tends to
be much more willy-nilly, making me think of the world of C libraries,
where people are much more likely to write their own library to do
something than to use someone else's, just because hooking things
together and finding libraries and the like is so painful.

findlib provides some nice features along these lines (though I think
it'd be nice if some of this functionality were folded into the
standard ocaml distribution, to encourage people to use it), but
without a discipline (community-driven, of course) for managing the
published module namespace, I don't think library development is
likely to grow like it has in Perl and Java-land--even with more
people developing.

In short, it's nice that ocaml has a compilation model that allows
more traditional models of building software than SML's CM package.
I'm much more comfortable with Makefiles and separate compilation than
with CM. But there *are* amenities that would make library creation,
publication, and use much more common.


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