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

[Phil Eaton <me@eatonphil.com>]

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Until you get a better response, I can point you in a direction. You want
to look up first-class modules
<https://realworldocaml.org/v1/en/html/first-class-modules.html>. It looks
like you will want something like this (taken from that page):

let to_int m =
    let module M = (val m : X_int) in
    M.x

Except yours will look more like:


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


Sorry about the awful formatting.


On Tue, Jun 16, 2015 at 8:15 AM, Rémy El Sibaïe Besognet <
remy.el-sibaie@lip6.fr> wrote:

> 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
>
>


-- 
Phil Eaton
http://eatonphil.com

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