propellor/doc/todo/info_propigation_out_of_nested_properties.mdwn

Currently, Info about a Host's Properties is manually gathered and
propigated. propertyList combines the Info of the Properties in the list.
Docker.docked extracts relevant Info from the Properties of the container
(but not al of it, intentionally!).

This works, but it's error-prone. Consider this example:

	withOS desc $ \o -> case o of
                (Just (System (Debian Unstable) _)) -> ensureProperty foo
		_ -> ensureProperty bar

Here, the Info of `foo` is not propigated out. Nor is `bar`'s Info.
Of course, only one of them will be run, and only its info should be propigated
out..

This commonly afflicts eg, privData. For example, `User.hasPassword'`
has this problem, and this prevents --list-fields from listing privdata
that's not set from that property.

One approach might be to make the Propellor monad be able to be run in two
modes. In one mode, it actually perform IO, etc. In the other mode, all
liftIO is a no-op, but all Info encountered is accumulated using a Reader
monad. This might need two separate monad definitions.

That is surely doable, but the withOS example above shows a problem with it --
the OS is itself part of a Host's info, so won't be known until all its
properties have been examined for info!

Perhaps that can be finessed. We don't really need to propigate out OS info.
Just DNS and PrivDataField Info. So info could be collected in 2 passes,
first as it's done now by static propertyInfo values. Then take that
and use it as the Info when running the Properties in the Reader monad.
Combine what the Reader accumulates with the static info to get the full
info.