# How to get a Caqti\_eio.stdenv from a Eio\_unix.Stdenv.base
**URL:** https://discuss.ocaml.org/t/how-to-get-a-caqti-eio-stdenv-from-a-eio-unix-stdenv-base/14183
**Category:** Ecosystem
**Tags:** eio
**Created:** [February 25, 2024, 8:15pm UTC](https://discuss.ocaml.org/t/how-to-get-a-caqti-eio-stdenv-from-a-eio-unix-stdenv-base/14183 "2024-02-25T20:15:16Z")
**Posts on this page:** 4
**Page:** 1
### Author:  [@Frederic\_Loyer](https://discuss.ocaml.org/u/Frederic_Loyer)
#### Post date: [February 25, 2024, 8:15pm UTC](https://discuss.ocaml.org/t/how-to-get-a-caqti-eio-stdenv-from-a-eio-unix-stdenv-base/14183/1 "2024-02-25T20:15:16Z")
`Caqti_eio` has a function which needs a parameter of type
```auto
type stdenv = <
net : [`Generic] Eio.Net.ty Eio.Std.r;
clock : float Eio.Time.clock_ty Eio.Std.r;
mono_clock : Eio.Time.Mono.ty Eio.Std.r;
>
```
If I feed it with a `Caqti_eio.stdenv` given by `Eio_main.run`, I have the error:
```plaintext
File "main.ml", line 9, characters 47-53:
9 | Caqti_eio.with_connection ~stdenv
^^^^^^
Error: This expression has type Eio_unix.Stdenv.base
but an expression was expected of type Caqti_eio.stdenv
The second object type has no method backend_id
```
I do understand that the object I give implements to many methods (`backend_id` is one of them). On most languages, there is an issue when there is not enough methods.
Is there a **simple** way to have a an object which match the type (just the 3 needed methods) based on `stdenv:Eio_unix.Stdenv.base`.
I have tried to patch `caqti`and add a `..` to tolerate other methods, but this doesn’t compile anymore:
```auto
65 | type stdenv = <
66 | net : [`Generic] Eio.Net.ty Eio.Std.r;
67 | clock : float Eio.Time.clock_ty Eio.Std.r;
68 | mono_clock : Eio.Time.Mono.ty Eio.Std.r;
69 | ..
70 | >
Error: A type variable is unbound in this type declaration.
In type
< clock : float Eio.Time.clock_ty Eio.Std.r;
mono_clock : Eio.Time.Mono.ty Eio.Std.r;
net : [`Generic] Eio.Net.ty Eio.Std.r; .. >
as 'a the variable 'a is unbound
```
(The error doesn’t print without the `..`).
---
### Author:  [@yawaramin](https://discuss.ocaml.org/u/yawaramin)
#### Post date: [February 25, 2024, 8:49pm UTC](https://discuss.ocaml.org/t/how-to-get-a-caqti-eio-stdenv-from-a-eio-unix-stdenv-base/14183/2 "2024-02-25T20:49:31Z")
Could you give a slightly more complete but still minimalistic code sample which we can use to try to debug the issue?
---
### Author:  [@Frederic\_Loyer](https://discuss.ocaml.org/u/Frederic_Loyer)
#### Post date: [February 25, 2024, 8:57pm UTC](https://discuss.ocaml.org/t/how-to-get-a-caqti-eio-stdenv-from-a-eio-unix-stdenv-base/14183/3 "2024-02-25T20:57:36Z")
I end up to (with a patched ppx\_rapper to support Caqti.2.x):
```auto
let create_query = [%rapper execute "CREATE TABLE t (a INTEGER, b INTEGER)"]
class caqti_env (stdenv:Eio_unix.Stdenv.base) = object
val s = stdenv
method net = s#net
method clock = s#clock
method mono_clock = s#mono_clock
end
let () =
match
Eio_main.run @@ fun stdenv ->
Caqti_eio.with_connection ~stdenv:(new caqti_env stdenv)
(Uri.of_string "sqlite3:essai.sqlite")
(function cnx ->
let* () = create_query () cnx in
Result.Ok ()
)
with
| Result.Ok () ->
output_string "OK\n"
| Result.Error err ->
output_string (Caqti_error.show err)
```
converting stdenv to `new caqti_env stdenv` did solved the method issue, but I find the method a little heavy. I guess I should work on a better way.
However, I get a new issue: the `stdenv#net` returns something of type
```auto
[`Generic | `Unix] Eio.Net.ty Eio.Resource.t
```
And the `Unix is not accepted by `Caqti_eio`.
EDIT: I had an simple answer here… [Eio: stdenv type not compatiable with Eio\_unix.Stdenv · Issue #114 · paurkedal/ocaml-caqti · GitHub](https://github.com/paurkedal/ocaml-caqti/issues/114) just type `stdenv :> Caqti_eio.stdenv`
But now, I have the error at the execution:
```auto
Failed to load driver for : Neither caqti-driver-sqlite3 nor the dynamic linker is linked into the application.
```
Curious. The Lwt version works fine, and there is the following stanza in my `dune` file:
```t
(libraries
ppx_rapper_eio
eio eio_main
caqti-eio
caqti-driver-sqlite3)
```
---
### Author:  [@talex5](https://discuss.ocaml.org/u/talex5)
#### Post date: [February 26, 2024, 11:30am UTC](https://discuss.ocaml.org/t/how-to-get-a-caqti-eio-stdenv-from-a-eio-unix-stdenv-base/14183/4 "2024-02-26T11:30:19Z")
It’s because `Caqti_eio.with_connection`’s signature requires something of type `Caqti_eio.stdenv` rather than ``. You could work around it by putting this at the start:
```ocaml
module Caqti_eio = struct
include Caqti_eio
let with_connection ~stdenv uri fn =
with_connection ~stdenv:(stdenv :> stdenv) uri fn
end
```
See the [Passing env](https://github.com/ocaml-multicore/eio?tab=readme-ov-file#passing-env) section of the README for more on this.