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Currently, a RevertableProperty's Properties always both HasInfo. This
means that if a Property NoInfo is updated to be a RevertableProperty, and
someplace called ensureProperty on it, that will refuse to compile.
The workaround is generally to export the original NoInfo property under
a different name, so it can still be used with ensureProperty.
This could be fixed:
data RevertableProperty i1 i2 where
RProp :: Property i1 -> Property i2 -> RevertableProperty i1 i2
However, needing to write "RevertableProperty HasInfo NoInfo" is quite
a mouthful!
Since only 2 places in the propellor source code currently need to deal
with this, it doesn't currently seem worth making the change, unless a less
intrusive way can be found.
Probably related would be to make RevertableProperty a constructor in the
Property GADT, which would allow more property combinators to work on
RevertableProperties. That would look like:
data Propety i where
...
RProp :: Property i1 -> Property i2 -> Property (CInfo i1 i2)
In this case, there's only one Info/NoInfo encompassing both sides, and
so ensureProperty could only be used on it if both sides were NoInfo.
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