Is "not_found" function not available?

[koji]

I cloned this opium repository.
and executed build (cd examples; dune build @examples)
No problem.

Then i started to create a new sample opium project.(to lean this framework)

1. create a new directory.
2. create dune file
3. create hello_world.ml

dune

(executables
 (libraries re ezjsonm opium logs.fmt opium_kernel)
 (preprocess
  (pps ppx_sexp_conv))
 (flags
  (:standard -safe-string))
 (names  hello_world))

(alias
 (name examples)
 (deps hello_world.exe))

hello_world.ml

open Opium.Std

let hello =
  get "/hello/" (fun _req ->
      `String ("Hello") |> respond')

let default =
  not_found (fun _req ->
      `Json Ezjsonm.(dict [("message", string "Route not found")]) |> respond')

let _ = App.empty |> default
  |> App.run_command

Then executed dune build hello_world.exe
But i got a following error at this time.

[koji:hello_opium]$ dune build hello_world.exe
File "hello_world.ml", line 8, characters 2-11:
8 |   not_found (fun _req ->
      ^^^^^^^^^
Error: Unbound value not_found
[koji:hello_opium]$

Is this expected behavior?
Is not_found function available only by example?

[Anurag Soni]

the not_found function was added recently and its not available on the version published on the opam registry. You can access it if you pin opium and opium_kernel to thee version in this repository.

opam pin add opium_kernel.dev --dev-repo
opam pin add opium.dev --dev-repo

[koji]

Thank you very much for your quickly helpful answer!
I could understand the situation.

[Anurag Soni]

You are welcome!