[Caml-list] polymorphic modules in classes

[Caml-list] polymorphic modules in classes

[Axel Simon]

Hi,

I would like to use a Hashtbl which is polymorphic in its elements. The 
elements should by compared by physical equality so I cannot use the 
"generic interface". My goal is to create a hash table over the type 'a 
which maps to int. I wrote:

module H = Hashtbl;;

module PhysEq : H.HashedType = struct
  type t 
  let equal = (==)
  let hash  = H.hash
end;;

module PhysHashtbl : H.S = H.Make(PhysEq);;

class ['a] bulkWriter ((fname, wr) : string * (out_channel -> 'a -> unit)) 
=
  let outCh = open_out_bin fname
  and stored   : (a' PhysHashtbl.t) = (PhysHashtbl.create 100)
  in object
  (* something here *)   
end

When I compile, I get "Unbound type constructor a'" for the line which 
introduces stored, although it works with int. I guess there are many 
other problems with my solution: PhysEq.t is abstract and PhysHashtbl.key 
is not bound to any type. How do I solve all this?

I am really new to OCaml and I couldn't make sense of the tutorials and 
other messages posted on this list.

Any help appreciated,
Axel.

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Re: [Caml-list] polymorphic modules in classes

[Jacques Garrigue]

From: Axel Simon <A.Simon@ukc.ac.uk>

> When I compile, I get "Unbound type constructor a'" for the line which 
> introduces stored, although it works with int.

This just looks like a typo: this should be 'a (a type variable), not
a' (an identifier). When fixed, your program is accepted.

> I guess there are many 
> other problems with my solution: PhysEq.t is abstract and PhysHashtbl.key 
> is not bound to any type. How do I solve all this?

This one is a much more serious problem, and appears frequently on
this list.
The only solution currently is to copy the Hashtbl module, replacing
key by 'a key, and 'a t by ('b,'a) t.

A simpler workaround is to define a functor producing your class.

module Make(T : sig type t end) = struct
  module PhysEq = struct
    type t = T.t
    let equal = (==)
    let hash  = H.hash
  end

  module PhysHashtbl : H.S with type key = T.t = H.Make(PhysEq)

  class ['a] bulkWriter ((fname, wr) : string * (out_channel -> 'a -> unit))  =
    let outCh = open_out_bin fname
    and stored   : ('a PhysHashtbl.t) = (PhysHashtbl.create 100)
    in object
     (* something here *)   
    end
end

Then you can create a class for every type you need.

But I'm not sure exactly of what you intend to use your hash table
for. Does it really need to handle many types?

Jacques Garrigue

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