I've got a server defined in config.hs as follows:

myserver :: Host
myserver = host "myserver.mydomain" $ props
    & standardSystem (Stable "jessie") X86_64 [ "Welcome to myserver!" ]

I'm writing a module (to deploy Matrix, FWIW) which has a section like this:

sources :: Property Debian
sources = File.hasContent "/etc/apt/sources.list.d/matrix.list"
            [ "# Deployed by Propellor"
            , ""
            , "deb http://matrix.org/packages/debian/ jessie main"
            ] `onChange` Apt.update

What I would like to be able to do, for example, is pull "jessie" from the standardSystem line into the sources function.

The host name is another I'd like to be able to pull in, so that I can abstract as much as possible and wind up with a line that looks not unlike this:

& Matrix.server 

Instead of

& Matrix.server hostname jessie

Am I barking up the wrong tree and should I just embrace the latter?