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

[Anton Lavrik <alavrik@piqi.org>]

Hi Francois,

I don't use .mli files that much. Granted, I'm a rather casual OCaml
user, but hey, at least you are not alone :)

I'm surprised by some of the comments you've received. The fact that
some people tend to practice top-down coding more than others doesn't
really mean anything. Other people do it differently even regardless
of the language they use. For me, paper and pencil are far more useful
than .mli files up until the interfaces converge and stabilize.

In general, .mli files are useful and even essential for libraries and
large projects. For instance, they allow to clearly (and cleanly)
define interfaces and help with separate compilation (i.e. to avoid
recompiling parts).

The biggest inconvenience with .mli files as I see it is that I have
to repeat myself and make related but slightly different changes in
two places when I change a module implementation. I would very much
prefer to declare and document public interfaces next to the
implementation and have language tooling take care of separate
compilation and documentation generation.

OCaml is kind of clumsy in this respect. For example, it does allow to
specify types for values and function parameters inline. The syntax
isn't the best, but the feature itself is very useful and I rely on it
all the time. But when I get to define a type signature for a function
e.g. in .mli file, I loose the ability to use parameter names and have
to specify them in the comments.

Overall, I count .mli files as a fairly minor language usability
issue. Perhaps, it wouldn't be even very hard to fix, for example, by
allowing something like "[public] val value-name :  typexpr" in .ml
files so that .mli/.cmi files can be generated automatically with
desired public interfaces.

Anton


On Mon, Oct 29, 2012 at 7:43 PM, Francois Berenger <berenger@riken.jp> wrote:
> Hello,
>
> Here is my stupid question of the day:
> what's the use of those .mli files?
>
> Is it just to separate interface from implementation
> so that the implementation of a module can be changed
> without clients of its interface to have to bother?
>
> Does it make compilation of large software faster
> by allowing for more parallelization and maybe later on avoiding to
> recompile some parts?
>
> Usually I program in a pure functional style, so my modules
> don't carry an internal state.
> I feel like "if someone want to re-use a function, so be it".
> If I really want to hide a function that I am afraid people
> may call in an incorrect manner, I declare it internally
> to some public function and use it correctly.
>
> Also, maybe I only work on toy-size OCaml projects. So, I never bothrered to
> create any .mli file.
> I would like to know if I should bother about them.
>
> Thanks a lot,
> Francois.
>
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