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Making a polymorphic type non-polymorphic to comply with original signature
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 Date: 2009-01-20 (17:05) From: Hugo Ferreira Subject: Re: [Caml-list] Making a polymorphic type non-polymorphic to comply with original signature
```Martin,

Thanks for the alternate solution. It seems
to provide a much simpler interface i.e:
doesn't require such a convoluted way of
declaring the types.

I wonder if it has any run-time benefits
compared to the previous solution.

Regards,
Hugo F.

Martin Jambon wrote:
> Hugo Ferreira wrote:
>> Hello,
>>
>> I have the following definition:
>>
>> module rec H :
>>   sig
>>       type 'a node =
>>           | Node of 'a node J.t
>>           | Leaf of 'a
>>
>>      type 'a t = 'a node
>>      val equal : 'a t -> 'a t -> bool
>>     val hash : 'a t -> int
>>   end =
>> struct
>>     type 'a node =
>>         | Node of 'a node J.t
>>         | Leaf of 'a
>>
>>     type 'a t = 'a node
>>     let equa = (==)
>>     let hash = Hashtbl.hash
>> end
>>
>> and J : Hashtbl.S with type 'a key = 'a H.node = Hashtbl.Make( H )
>> ;;
>>
>> The data type 'a node is polymorphic. It is also a key used by the
>> hash-table. Note that H now does not comply with Hashtbl.HashedType
>> (because Hashtbl.HashedType is not polymorphic). By adding the
>> constraint "with type" also does not help.
>>
>> Is it possible to make H comply with Hashtbl.HashedType i.e: make
>> J.Key = 'a H.node ?
>
>
> I just posted an implementation of polymorphic hash tables with physical
> comparison, using the Obj module:
>
>   http://martin.jambon.free.fr/phys.html
>
> It is the following code:
>
> (***** phys.mli *****)
>
> type ('a, 'b) t
>
> val add : ('a, 'b) t -> 'a -> 'b -> unit
> val clear : ('a, 'b) t -> unit
> val copy : ('a, 'b) t -> ('a, 'b) t
> val create : int -> ('a, 'b) t
> val find : ('a, 'b) t -> 'a -> 'b
> val find_all : ('a, 'b) t -> 'a -> 'b list
> val fold : ('a -> 'b -> 'c -> 'c) -> ('a, 'b) t -> 'c -> 'c
> val iter : ('a -> 'b -> unit) -> ('a, 'b) t -> unit
> val length : ('a, 'b) t -> int
> val mem : ('a, 'b) t -> 'a -> bool
> val remove : ('a, 'b) t -> 'a -> unit
> val replace : ('a, 'b) t -> 'a -> 'b -> unit
>
>
>
> (***** phys.ml *****)
>
> (*
>   This implementation uses the Obj module which allows to transgress
>   the type system. It is not regular OCaml.
> *)
>
> module Magic_key =
> struct
>   type t = Obj.t
>   let equal = ( == )
>   let hash = Hashtbl.hash
> end
>
> module Magic_table = Hashtbl.Make (Magic_key)
>
>
> type ('a, 'b) t = 'b Magic_table.t
>
> let add tbl k v = Magic_table.add tbl (Obj.repr k) v
> let clear tbl = Magic_table.clear tbl
> let copy tbl = Magic_table.copy tbl
> let create n = Magic_table.create n
> let find tbl k = Magic_table.find tbl (Obj.repr k)
> let find_all tbl k = Magic_table.find_all tbl (Obj.repr k)
> let fold f tbl accu = Magic_table.fold (Obj.magic f) (Obj.magic tbl) accu
> let iter f tbl = Magic_table.iter (Obj.magic f) (Obj.magic tbl)
> let length tbl = Magic_table.length tbl
> let mem tbl k = Magic_table.mem tbl (Obj.repr k)
> let remove tbl k = Magic_table.remove tbl (Obj.repr k)
> let replace tbl k v = Magic_table.replace tbl (Obj.repr k) v
>
>
> (*****************)
>
>
>
> Martin
>

```