Propellor.Property.Versioned

Versioned properties and hosts.

When importing and using this module, you will need to enable some language extensions:

{-# LANGUAGE RankNTypes, FlexibleContexts, TypeFamilies #-}

This module takes advantage of RevertableProperty to let propellor switch cleanly between versions. The way it works is all revertable properties for other versions than the current version are first reverted, and then propellor ensures the property for the current version. This method should work for any combination of revertable properties.

For example:

 demo :: Versioned Int (RevertableProperty DebianLike DebianLike)
 demo ver =
 	ver (   (== 1) --> Apache.modEnabled "foo"
		`requires` Apache.modEnabled "foosupport"
	    <|> (== 2) --> Apache.modEnabled "bar"
 	    <|> (> 2)  --> Apache.modEnabled "baz"
 	    )

 foo :: Host
 foo = host "foo.example.com" $ props
 	& demo `version` (2 :: Int)

Similarly, a whole Host can be versioned. For example:

 bar :: Versioned Int Host
 bar ver = host "bar.example.com" $ props
	& osDebian Unstable X86_64
 	& ver (   (== 1) --> Apache.modEnabled "foo"
 	      <|> (== 2) --> Apache.modEnabled "bar"
 	      )
 	& ver ( (>= 2) --> Apt.unattendedUpgrades )

Note that some versioning of revertable properties may cause propellor to do a lot of unnecessary work each time it's run. Here's an example of such a problem:

 slow :: Versioned Int -> RevertableProperty DebianLike DebianLike
 slow ver =
 	ver (   (== 1) --> (Apt.installed "foo" <!> Apt.removed "foo")
	    <|> (== 2) --> (Apt.installed "bar" <!> Apt.removed "bar")
        )

Suppose that package bar depends on package foo. Then at version 2, propellor will remove package foo in order to revert version 1, only to re-install it since version 2 also needs it installed.

Documentation

type Versioned v t = VersionedBy v -> t

Something that has multiple versions of type v.

version :: Versioned v t -> v -> t

Access a particular version of a Versioned value.

(-->) :: (v -> Bool) -> RevertableProperty metatypes metatypes -> VerSpec v metatypes infixl 8

Specify a function that checks the version, and what RevertableProperty to use if the version matches.

(<|>) :: VerSpec v metatypes -> VerSpec v metatypes -> VerSpec v metatypes infixl 2

Add an alternate version.