Re: [Caml-list] Type inference + optional parameters

[Jacques Garrigue <garrigue@kurims.kyoto-u.ac.jp>]

From: Alain.Frisch@ens.fr
> On Wed, 3 Sep 2003, Jacques Garrigue wrote:
> > From: Christoph Bauer <c_bauer@informatik.uni-kl.de>
> >
> > > ocamls type inference uses information of optionl arguments. This
> > > results in a strange behaviour.
> >
> > Brian Rogoff already answered this one. Do you seriously expect a
> > type-safe compiler to simply ignore the type of the default value :-)
> 
> I guess the original question is not that naive. It would be possible to
> have a type system with an extension like your multimatch, to make the
> type of the result of a function depend on the presence/absence of
> optional argument.
> 
> With a crazy syntax to express conditional unification constraints, you
> could write for instance:
> 
> do_with_opt_conv: ?conv:('a -> 'b  orelse 'b = 'a) -> 'a -> 'b
> 
> meaning that in the absence of the optional argument, the result type
> must be unified with the argument type (there are more constraints
> when the optional argument is not provided).
> 
> Does it make sense ?

It makes sense, and is just a direct consequence of the Default module
I was describing in my mail. I've already considered it 4 years ago,
after a comment for Wolfram Kahl.
However, if you let this behaviour be active with all default
parameters, you end up with overly general types; usually you expect
the concrete argument to be of the same type as the default.
Moreover, there may be hidden pitfalls: type incompatibilities you
only detect later, when several optional parameters are omitted
simultaneously.

All in all, I think that this feature is just on the borderline of
practical typing: possible, but the typing is too complex for casual
use, and may result in confusing type errors.
In some cases being explicit simplifies lots of things...

Jacques Garrigue

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