94 lines
2.8 KiB
Markdown
94 lines
2.8 KiB
Markdown
Currently, Info about a Host's Properties is manually gathered and
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propigated. propertyList combines the Info of the Properties in the list;
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Docker.docked extracts relevant Info from the Properties of the container
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(but not al of it, intentionally!); etc.
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This works, but it's error-prone. Consider this example:
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withOS desc $ \o -> case o of
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(Just (System (Debian Unstable) _)) -> ensureProperty foo
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_ -> ensureProperty bar
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Here, the Info of `foo` is not propigated out. Nor is `bar`'s Info. Of
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course, only one of them will be run, and only its info should be
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propigated out..
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----
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Currently, ensureProperty detects if it's called on a property with a
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non-empty Info, and prints a warning. Would prefer to handle this at the
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type level though..
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----
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One approach might be to make the Propellor monad be able to be run in two
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modes. In run mode, it actually performs IO, etc. In introspection mode, all
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liftIO is a no-op, but all Info encountered is accumulated using a Reader.
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This might need two separate monad definitions.
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That is surely doable, but consider this example:
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property "demo" = do
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needfoo <- liftIO checkFoo
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if needfoo
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then ensureProperty foo
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else ensureProperty bar
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In introspection mode, the liftIO is a no-op, but needs to return a Bool.
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That seems unlikely (how to pick which?), but even if some defaulting is
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used, only one of foo or bar's info will be seen.
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----
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Another approach could be something like this:
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withInfoFrom foo $ \callfoo ->
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withInfoFrom bar $ \callbar ->
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property "demo" = do
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needfoo <- liftIO checkFoo
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if needfoo
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then callfoo
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else callbar
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Here withInfoFrom is able to add foo and bar's Info to the info of the
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property that (may) call them.
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This approach is not fully type safe; it would be possible to call
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withInfoFrom in a way that didn't let it propigate the info.
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Also it has the problem that it doesn't support this:
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property "demo" = do
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needfoo <- liftIO checkFoo
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if needfoo
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then do
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foop <- liftIO getFooParam
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ensureProperty (foo foop)
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else ensureProperty bar
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----
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Another approach would be to add a new SimpleProperty, which is a property
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that has no Info. Only allow calling ensureProperty on this new type.
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(Or, remove propertyInfo from Property, and add a new InfoProperty that
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has the info.)
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This would surely work, but it adds a lot of complexity to property
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combiators. Also, propertyList could only contain one type at a time,
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not a mixed list of Property and SimpleProperty.
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Could a GADT be used instead?
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{-# LANGUAGE GADTs #-}
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{-# LANGUAGE EmptyDataDecls #-}
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data HasInfo
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data NoInfo
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data Property i where
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InfoProperty :: Desc -> Propellor Result -> Info -> Property HasInfo
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SimpleProperty :: Desc -> Propellor Result -> Property NoInfo
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ensureProperty :: Property NoInfo -> Propellor Result
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ensureProperty (SimpleProperty d r) = r
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