How about something like the follwing--the idea coming from 
posts I read a while ago about CPS programming:

module Scan : sig

(* type 'a set *)

type ('a, 'b) conversion  = ( 'b -> 'a ) -> 'a

(* the following signatures refer to stdin scanners *)
val s  : (string -> 'a) -> 'a
val d  : (int    -> 'a) -> 'a
val i  : (int    -> 'a) -> 'a
val o  : (int    -> 'a) -> 'a
val x  : (int    -> 'a) -> 'a
val f  : (float  -> 'a) -> 'a
val c  : int -> (string -> 'a) -> 'a
(* val set: char set -> (string -> 'a) -> 'a *)

type ('a, 'b) scanner = ( 'b -> 'a ) -> ('a, 'c) scanner
val nil :  (unit, unit) scanner

val (+)  : 'a scanner -> ('a, 'b) conversion -> 'a scanner

val get  : ('a, 'b) scanner -> 'a list

end

Basically, a scanner is a fifo of conversions. Scanners are 
built by +ing a scanner (left operand) and a conversion 
(right operand). The fifo must be initialized with a nil 
scanner, containing no conversions. After a scanner has been 
built, it must be applied self-recursively to a succession 
of parameters, each representing a callback for a conversion 
  in the fifo of the scanner. Because a scanner has a 
recursive type, the result of its application to a callback 
is a scanner with one less convertions in its fifo. As the 
convertions are performed, each the result of each callback 
is appended to a list maintained internally by the scanner. 
Finally, when all conversions have been performed, the 'a 
list built by the scanner can be gotten with the get function.

Of course, all this works in my best dreams. The scanner 
type cannot be defined as I have because of the unbound type 
parameter 'c. But there are work arounds, although not 
terribly elegant. For example, we could define a big variant
type callback_parameter = String of string | Int of int | 
Float of float | ...
but this would greatly reduce the beauty of such a solution.

At 3:36 in the morning I can do no better than this. Pierre, 
Xavier, your turn.

Alex

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