Re: [Caml-list] Why should I use .mli files?

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

Wow.
I see that a big debate is going on mli files and declarations
inside ml files.

I think that lots of people have given this problem a lot of thought
during many years.
My conclusion has unfortunately been that trying to solve this at
the language level without breaking backward compatibility is
very difficult.

For instance, one is currently allowed to do something like that:

a.ml:
let f x = x

let b = f true

a.mli:
val f: int -> int

This may look stupid, but this kind of code (of course more
complicated) exists in the wild, and sometimes for sensible
reasons.
For this kind of reason, I do not see any easy way to import
information from mli files to ml files.

Now, is maintaining mli files really painful?
In my experience, not at all.
Taking as example the ocaml compiler itself, the only
duplication that bothers me a little is the error type exported
by many modules, as one always has to keep it in sync by
copy-paste. This is a bit of a problem because for most uses
this type could be abstract (you don't really care about its
contents), but in some rare occasions you need it public.
For other types,  do not see syncing as a disadvantage, because
it rather makes you conscious that other modules depend on
this type, so better look at it twice :-)

Of course I know that some people (probably with a different
background) do not like mli files.
Couldn't we just imagine a preprocessing tool that allows to
generate both ml and mli from the same file?
This is pretty standard for literate programming.
It could even call the compiler if you want the types to be inferred
automatically (even though I personally think that having to write
value types in the mli file is good, because you know when you
are changing an export.)
As an external tool, it could use pragmas (keywords inside
comments) for instance.
Something close to ocamldoc (ocamldoc itself?) but which you
would integrate in the compilation cycle.

Anyway, I just wanted to state, from my experience, my lack of
enthusiasm about modifying the core language to accommodate
that.

Jacques Garrigue