RE: [Caml-list] Polymorphism and the "for" loop

["Harrison, John R" <johnh@ichips.intel.com>]

| (Incidentally, O'Caml doesn't really need a type "void" since it can
convey
| a similar semantic notion using polymorphism.  Consider the function
raise,
| which has type "exn -> 'a" -- in other words, given an exception, it
| produces an arbitrary value of any type.  Since such a thing is
impossible
| (using the type-safe core language), we must conclude that raise does
not
| return to its call site when it executes.)

That's true. In any case I assume (given that a recursive type is some
kind of least fixed point) that you can define an empty type in OCaml
like
this:

  # type void = Constr of void;; 
  type void = Constr of void

So the only functions into such a type are those that never terminate
and return a value, e.g.

  # let rec f x = (f:int->void) x;;
  val f : int -> void = <fun>

or

  # let (g:int->void) = fun x -> failwith "void function";;
  val g : int -> void = <fun>

or

  # let (h:int->void) = fun x -> if x = 0 then f x else g x;;
  val h : int -> void = <fun>

Perhaps the name "void" in C is a bit poorly chosen. It's probably meant
to say that an element of the type imparts no information, rather than
imply a semantic model based on an empty type.

John.

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