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

    -- | 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.

    module Propellor.Property.Versioned (Versioned, version, (-->), (<|>)) where

    import Propellor
    import Propellor.Types.Core

    import Data.List

    -- | Something that has multiple versions of type `v`.
    type Versioned v t = VersionedBy v -> t

    type VersionedBy v
    	= forall metatypes. Combines (RevertableProperty metatypes metatypes) (RevertableProperty metatypes metatypes)
    	=> (CombinedType (RevertableProperty metatypes metatypes) (RevertableProperty metatypes metatypes) ~ RevertableProperty metatypes metatypes)
    	=> (VerSpec v metatypes -> RevertableProperty metatypes metatypes)

    -- | Access a particular version of a Versioned value.
    version :: (Versioned v t) -> v -> t
    version f v = f (processVerSpec v)

    -- A specification of versions.
    --
    -- Why is this not a simple list like
    -- [(v -> Bool, RevertableProperty metatypes metatypes)] ?
    -- Using a list would mean the empty list would need to be dealt with,
    -- and processVerSpec does not have a Monoid instance for
    -- RevertableProperty metatypes metatypes in scope, and due to the way the
    -- Versioned type works, the compiler cannot find such an instance.
    --
    -- Also, using this data type allows a nice syntax for creating
    -- VerSpecs, via the `<&>` and `alt` functions.
    data VerSpec v metatypes
    	= Base (v -> Bool, RevertableProperty metatypes metatypes)
    	| More (v -> Bool, RevertableProperty metatypes metatypes) (VerSpec v metatypes)

    processVerSpec
    	:: Combines (RevertableProperty metatypes metatypes) (RevertableProperty metatypes metatypes)
    	=> (CombinedType (RevertableProperty metatypes metatypes) (RevertableProperty metatypes metatypes) ~ RevertableProperty metatypes metatypes)
    	=> v
    	-> VerSpec v metatypes
    	-> RevertableProperty metatypes metatypes
    processVerSpec v s = combinedp s
    	`describe` intercalate " and " (combineddesc s [])
      where
    	combinedp (Base (c, p))
    		| c v = p
    		| otherwise = revert p
    	combinedp (More (c, p) vs)
    		| c v = combinedp vs `before` p
    		| otherwise = revert p `before` combinedp vs
    	combineddesc (Base (c, p)) l
    		| c v = getDesc p : l
    		| otherwise = getDesc (revert p) : l
    	combineddesc (More (c, p) vs) l
    		| c v = getDesc p : combineddesc vs l
    		| otherwise = getDesc (revert p) : combineddesc vs l

    -- | Specify a function that checks the version, and what
    -- `RevertableProperty` to use if the version matches.
    (-->) :: (v -> Bool) -> RevertableProperty metatypes metatypes -> VerSpec v metatypes
    c --> p = Base (c, p)

    -- | Add an alternate version.
    (<|>) :: VerSpec v metatypes -> VerSpec v metatypes -> VerSpec v metatypes
    Base a <|> Base b = More a (Base b)
    Base a <|> More b c = More a (More b c)
    More b c <|> Base a  = More a (More b c)
    More a b <|> More c d = More a (More c (b <|> d))

    infixl 8 -->
    infixl 2 <|>