Re: [Caml-list] Unifying buildt-in types with polymorphic types

[Jeremy Yallop <yallop@gmail.com>]

On 15 May 2014 20:24, Peter Frey <pjfrey@sympatico.ca> wrote:
> I need all streams to have the same signature 'StreamOps'.

It's not possible with the current definition of StreamOps.  The
difficulty is that the stream type and the element type vary
non-uniformly in your various stream implementations:

   StmLzy defines streams of type 'a t = Cons of 'a * 'a t Lazy.t
                  with elements of 'a

   StmAry defines streams of type 'a t = int * 'a cursor
                  with elements of 'a

   StmStr defines streams of type 'a t = int * 'a cursor
                  with elements of char

but the signature leaves only the stream type 'a t unspecified,
providing no way for the element type to vary in the implementation.

One solution is to change the StreamOps signature to include both the
element type and the stream type.  Once you've added an element type
to the signature there's no need to parameterize the stream type:

   module type StreamOps = sig
     type elt (* the element type *)
     type t   (* the stream type *)
     val head : t -> elt
   end

The implementations of StreamOps then become functors over the element
type.  This is the approach used in the standard library to define
maps and sets (http://caml.inria.fr/pub/docs/manual-ocaml/libref/Set.Make.html).
 For example, you might define LazyStream as follows

   module LazyStream (E : sig type elt end)
      : StreamOps with type elt = E.elt =
   struct
     type elt = E.elt
     type t = Cons of elt * t Lazy.t
     let head (Cons (h, _)) = h
   end

The element type of StmStr doesn't vary, so there's no need to make it
a functor:

   module StmStr : StreamOps with type elt = char =
   struct
     type elt = char
     type cursor = { ofs: int; lim: int; data: string }
     type t = int * cursor
     let get s i = String.get s.data i
     let head (i, s) = get s i
   end

I hope that helps,

Jeremy.