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Date: Mon, 05 Mar 2001 16:10:56 -0800
From: Chris Hecker <checker@d6.com>
Subject: Re: [Caml-list] dynamically loading C functions
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>You can provide combinators to build the type information:

Wow, that's pretty cool.  I'm not that familiar with abstract types and modules in caml (or any functional language), so this was kind of an eye opener.  I think I understand how they work a bit more now, thanks.  I do have one thing I can't figure out.  Here's my test module:

module C:
    sig
      type 'a t
      val int_t   : int t
      val float_t : float t
      (* Begins with @ so it's right-associative :-) *)
      val (@->)   : 'a t -> 'b t -> ('a -> 'b) t
      val get_function : string -> 'a t -> 'a

      val conv : 'a t -> int list
    end =
  struct
    type 'a t = T of int | Tl of int list
    let int_t = T 1
    let float_t = T 2
    let (@->) a b = match a,b with 
      (T a,T b) -> Tl [a;b]
    | (T a,Tl b) -> Tl (a :: b)
    | (Tl a,Tl b) -> Tl (a @ b)
    | (Tl a,T b) -> Tl (a @ [b])
    let get_function s l = Obj.magic s (* just get it to compile *)
        
    let conv (Tl a) = a
  end

The way I've done 'a t is as a variant int list.  This works, but my @-> flattens nested lists because of the pattern match, so while the type system differentiates between

# (float_t @-> int_t @-> float_t @-> float_t @-> int_t);;
- : (float -> int -> float -> float -> int) C.t = <abstr>

and (note the nested parens):

# (float_t @-> (int_t @-> float_t @-> float_t) @-> int_t);;
- : (float -> (int -> float -> float) -> int) C.t = <abstr>

my internal representation doesn't:

# conv (float_t @-> int_t @-> float_t @-> float_t @-> int_t);;
- : int list = [2; 1; 2; 2; 1]

# conv (float_t @-> (int_t @-> float_t @-> float_t) @-> int_t);;
- : int list = [2; 1; 2; 2; 1]

I thought I could fix this by saying type 'a t = T of int | Tl of 'a t list, but that seems to constrain (@->) to  be 'a t -> 'a t -> 'a t and I lose the cruicial ('a -> 'b) t combinator (not sure if I'm using that word correctly)  and I get a type error.  Any idea how I could get it to nest?

Chris


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