Propellor's config file is written in Haskell, and Haskell is invaluable to extend Propellor with your own custom properties. But you don't need to know about monads to configure Propellor!

Let's take a quick tour of the config.hs file..

-- This is the main configuration file for Propellor, and is used to build
-- the propellor program.

So, -- starts a comment in this file.

import Propellor
import qualified Propellor.Property.File as File
import qualified Propellor.Property.Apt as Apt
import qualified Propellor.Property.User as User
import qualified Propellor.Property.Cron as Cron

This loads up Propellor's modules. You'll almost certainly want these; many more can be found in the API documentation.

main :: IO ()
main = defaultMain hosts

This config file is the Propellor program, and so it needs a little stub to go run itself. No need to ever change this part. hosts is the list of hosts that you configure, and it comes next:

-- The hosts propellor knows about.
hosts :: [Host]
hosts =
    [ mylaptop
    , myserver

Finally, you need to define the configuration for each host in the list:

mylaptop :: Host
mylaptop = host "" $ props
    & osDebian Unstable X86_64
    & Apt.stdSourcesList

myserver :: Host
myserver = host "" $ props
    & osDebian (Stable "stretch") X86_64
    & Apt.stdSourcesList
    & Apt.installed ["ssh"]

The configuration for the mylaptop host first tells propellor what OS it's running. Then the stdSourcesList line tells propellor to configure its /etc/apt/sources.list, using its OS. (Of course you might want to change that Unstable to Stable.)

Each property of the host is prefixed with an "&" operator. This just makes a list of properties.

Some other properties you may find in your config.hs, or want to add:

        & Apt.unattendedUpgrades
        & User.hasSomePassword (User "root")
        & "/etc/default/foodaemon" `File.containsLine` "ENABLED=yes"
        & Cron.runPropellor (Cron.Times "30 * * * *")

Some of these properties can be reverted -- this makes Propellor undo whatever effects they might have. For example, unattended upgrades can be scary, so maybe you turned that on, but want to disable it now. To do so, just change the "&" to a "!"

        ! Apt.unattendedUpgrades

Some properties cannot be reverted. Yet. It takes coding to implement revertability. If you try to revert a property that does not support reversion, propellor will fail to compile! This is a good thing.. it avoids you getting confused or bad things happening.

The error message when this happens might look a little scary. But if you read through it, it's remarkably precise about what and where the problem is.

    Couldn't match expected type `RevertableProperty'
                with actual type `Property DebianLike'
    In the return type of a call of `Apt.installed'
    In the second argument of `(!)', namely `Apt.installed ["ssh"]'
    In the first argument of `(&)', namely
      `host "" & Apt.stdSourcesList Unstable
       & Apt.unattendedUpgrades
       ! Apt.installed ["ssh"]'

Similarly, if you make a typo in the config file, you'll probably get a long but informative error message.

    Not in scope: `Apt.standardSourcesList'
    Perhaps you meant one of these:
      `Apt.stdSourcesList' (imported from Propellor.Property.Apt)

That's really all there is to configuring Propellor. Once you have a config.hs ready to try out, you can run propellor --spin $host on one of the hosts configured in it.

See the README for a further quick start and Writing Properties for guidance on extending propellor with your own custom properties.

(If you'd like to learn a little Haskell after all, check out Learn You a Haskell for Great Good.)