Virgile Prevosto [Tuesday 17 December 2002] :
 >
 > Hello,
 > Nobuyuki Tomizawa a écrit:
 > 
 > > ------------------------------------------------------------
 > > class foo s = object
 > >   val str : string = s
 > >   method to_string ?(opt = "" ) () = opt ^ s
 > > end;;
 > > 
 > > let l = [new foo "a"; new foo "b"; new foo "c" ];;
 > > 
 > > List.iter (fun e -> print_endline (e#to_string ())) l;;
 > > 
 > > File "test.ml", line 8, characters 52-53:
 > > This expression has type foo list but is here used with type
 > >   < to_string : unit -> string > list
 > > Type foo = < to_string : ?opt:string -> unit -> string >
 > > is not compatible with type < to_string : unit -> string > 
 > > ------------------------------------------------------------
 > 
 > This is quite normal: since the type of e is not constrained, ocaml
 > has inferred the most general one from the body of the function: e
 > must be an object with a method 'to_string' of type unit -> string
 > . The problem comes from the fact that optional arguments and type
 > inference do not work very well together (cf
 > http://pauillac.inria.fr/ocaml/htmlman/manual006.html section
 > 4.1.2).
 > As suggested in the manual, the best solution might be to add a
 > type annotation in the function above:
 > 
 > List.iter (fun (e:foo) -> print_endline (e#to_string ())) l;;
 > or
 > List.iter (fun (e:#foo) -> print_endline (e#to_string ())) l;;
 > if you intend to use subclasses of foo

Alternatively, one can use an "unwrapped" argument :

# List.iter (fun e -> print_endline (e#to_string ?opt:None ())) l;;
a
b
c
- : unit = ()

-- 
   Olivier
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