Le Sunday 11 April 2004 15:16, Jacques Garrigue a écrit :
> Basically, what you are asking for is a new type of constraint, which
> is only to be applied when the optional argument is omitted (and as
> result the default is selected).
> This is not possible in the current type system, and while there are
> some uses for that, it would be hard to justify making types more
> complex in the general case, just to handle this specific problem.
I would like to point out that a machinery to handle this problem
is guarded recursive datatypes proposed by Xi. It is a framework
that solves a bunch of other problems and is compatible with ml type
system (there is currently at Inria a student working on an Ocaml integration 
of this type system).
Combining it with your labeled parameters leads to a well typed code
analogous to your phantom-type based solution. Some even view
Xi's GRDT as a way to avoid your Obj.magic when matching against 
a phantom type. 
For those interested, the pseudo code would be something like :

guarded 'a opt = Omit with 'a = int | Arg of 'a->unit
let plot : ?(labels:'a opt) -> 'a graph -> unit =
  fun ?(labels=Omit) g ->
  let printing_labels : 'a -> unit = 
     match labels with 
       Omit -> print_int
    | Arg x -> x
  in
  ... printing_labels node_label ...

Nadji

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