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

[Peter Frey <pjfrey@sympatico.ca>]

On Thu, 2014-05-15 at 21:28 +0100, Jeremy Yallop wrote:
> 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.
> 
This helped a lot, thank you.  
(Typing is hard; I have been programming since 1963 and always had a
clear idea what I was doing, because previous languages translated
statements into code; Assemblers (many) Cobol Fortran Snobol Prolog
Pascal Cs ... Now the type checker translates types into types...
perhaps I am getting to old...)
thanks again
Peter