[ocaml-platform] on the need and design of OCaml namespaces

[Leo White]

On Feb 26 2013, Yaron Minsky wrote:

>Leo, do you have a summary of what you don't like about Alain's
>namespace proposal?  I'm not presently able to identify any obvious
>weaknesses in it.

I have a few of issues with Alain's proposal specifically, and then a 
second group of issues that are with that kind of proposal in general (i.e. 
defining the namespace separately from the modules it contains).

Issues with Alain's proposal specifically:

 1. Alain's proposal does not include a notion of opening a namespace. The 
    ability to only open a namespace when actually using it (e.g. a local 
    open) would be very useful. For example, this means that the proposal 
    provides no assistance for handling multiple standard libraries in the 
    same program: you can either make "List" equal "Core_List" everywhere or
    you can make it equal "Stdlib_List" everywhere. I see no real reason 
    why Alain's namespaces could not be extended with a namespace opening 
    feature.

 2. Alain's proposal provides no means of aliasing namespaces. Similar to 
    opening a namespace, it would be useful to be able to say "open 
    Core.Std as CS" and then refer to Core_Std_Mutex as "CS#Mutex"

 3. Alain's proposal only supports a flat namespace. As I said before, 
    hierarchical namespaces are very useful in certain situations. 
    Obviously this criticism only makes sense if you first add the ability 
    to open namespaces. Again this feature could easily be added to Alain's 
    proposal.

Issues with this kind of proposal:

 1. They require unique file names. This means that every file must become 
    "package_Module.ml" or "package_Subpackage_Module.ml".

 2. A module's name depends on other files. A module's name (by which I 
    mean its namespaced name) depends on the ".ns" file, this could 
    definitely be confusing.

 3. They require you to manage an ".ns" file. Not a major burden, but it is 
    one more place to look for errors. It also must be included in all 
    calls to the compiler.

 4. To me they put the namespace information in the wrong place. A 
    component should define its own names, it should also not have to open 
    its own namespace.

I think that the first group of issues above are important, but the second 
group of issues are of a more aesthetic nature. Also, if you want to 
automatically open modules along with a namespace, then that is much easier 
with centralised definition of the namespace.

As a side note, if people are okay with long filenames, why not support 
having filenames like "core-std-mutex.mli" and then allow "open namespace 
Core" and "open namespace Core#Std".

These could be treated as if there were automatically a "core.ns":

 namespace Std = "core-std.ns"

and "core-std.ns":

 module Mutex = "core-std-mutex.mli"

that were passed to any compiler invocation that had such files on its 
search path.