[Caml-list] Generic usage of Hashtbl.S with first class module

["Rémy El Sibaïe Besognet" <remy.el-sibaie@lip6.fr>]

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Hi all,

Considering this function

let find ht x =
  Hashtbl.find ht x


I would like to write a more general `find` function which takes the module
as first class module in parameter with the possibility to use it on
different implentations of Hashtbl.S.

let find (module H : Hashtbl.S) ht x =
  H.find *ht* x


but it failed with this error (on the underlined identifier) :

*Error: This expression has type 'a H.t but an expression was expected of
type         'a H.t       The type constructor H.t would escape its scope*



I also tried by specifying the implementation but had the same problem:

let find (type a) (module H : Hashtbl.S with type key = a) (ht : 'a
Ht.t) (x : a) =

H.find ht x

Anyone has an idea how to achieve this thing ?

Thanks,
- Rémy El Sibaïe, LIP6



PS: at the begining, I was trying on a memoization function which has more
interest than the previous function :

let memo_rec (module H : Hashtbl.S) h f =
  let rec g x =
    try H.find h x with
    | Not_found ->
      let y = f g x in
      H.add h x y; y
  in g

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