[Nix-dev] NixOS require attribute

[Nicolas Pierron]

Hi,

On Mon, May 9, 2011 at 12:19, Sander van der Burg - EWI
<S.vanderBurg at tudelft.nl> wrote:
> It seems that it is only possible to specify files in the require attribute
> of a Nix module, e.g.:
>
> {pkgs, ...}:
>
> {
>   require = [ /some/nixos/module/file.nix ]; # Works fine
>
>   services.openssh.enable = true;
> }
>
> But it seems that it is impossible to specify functions or attribute sets,
> e.g.:
>
> {pkgs, ...}:
>
> {
>   require = [ { services.httpd.enable = true; } ]; # Triggers an error
>
>   services.openssh.enable = true;
> }

This is a feature, in fact the error message you are getting should
look like "Option definitions used inside option declaration section".
 The reason was to accept old modules which are still written in the
syntax of the previous module system.  The syntax was to declare
options as a separated attribute set which was added in the require
list.  The code which unifies module syntax makes the assumption that
only one attribute set is used in the require attribute and that it
contains only option declarations.

The plan was to remove such possibilities in the future because this
we cannot provide good error messages out of such expression due to:

{pkgs, ...}:

{
  require = [ (import /some/nixos/module/file.nix) ];

  services.openssh.enable = true;
}

> I'm curious whether it is possible to also require functions or attribute
> sets. The reason I'm asking this is, that I'm working on a function that
> generates a network of NixOS configurations running web applications. In
> some cases you may want to run a database server and web server on a single
> machine, in other cases you may want to run a database and web server on
> separate machines.

It could be possible, but doing so will prevent the usage of good file
location in the error messages.

> I think using the require attribute is the best way to combine certain
> properties into a single NixOS configuration.

The require is indeed the best and only way to combine separated
concerns into a NixOS configuration, but you cannot use attribute sets
in them, and even if you would be able, the error reports would get
worst.

{pkgs, ...}:

{
  require = [ ./database.nix ./webserver.nix ];

  services.openssh.enable = true;
}

So the question is how hard is it to generate multiple files, and
should we accept attribute sets even if we get poor error messages?  I
don't think this goes to the right direction.

-- 
Nicolas Pierron
http://www.linkedin.com/in/nicolasbpierron - http://nbp.name/