Hello world example on Linux gives "Error: I cannot find the root of the current workspace/project"

[ndchorley]

I'm following Building a Hello World program, on Linux.

$ ls
dune  hello_world.ml

$ dune --version
3.0.3

$ cat dune
(executable
  (name hello_world))

$ cat hello_world.ml 
print_endline "Hello, world!"

Finally, trying to build gives:

$ dune build hello_world.exe
Error: I cannot find the root of the current workspace/project.
If you would like to create a new dune project, you can type:

    dune init project NAME
...

What am I doing wrong here?

Thanks.

[rgrinberg]

Do you have a project file? You need to have a dune-project file with the following contents at minimum:

(lang dune 3.1)