The problem this exposes has to do with requires. As implemented,
requires yields either a Property HasInfo or a Property NoInfo depending
on its inputs. That works. But look what happens when it's used:
*Propellor.Types> let foo = IProperty "foo" (return NoChange) mempty mempty
*Propellor.Types> let bar = IProperty "bar" (return NoChange) mempty mempty
*Propellor.Types> foo `requires` bar
<interactive>:17:5:
No instance for (Requires (Property HasInfo) (Property HasInfo) r0)
arising from a use of `requires'
The type variable `r0' is ambiguous
Possible fix: add a type signature that fixes these type variable(s)
Note: there is a potential instance available:
instance Requires
(Property HasInfo) (Property HasInfo) (Property HasInfo)
-- Defined at Propellor/Types.hs:167:10
Possible fix:
add an instance declaration for
(Requires (Property HasInfo) (Property HasInfo) r0)
In the expression: foo `requires` bar
In an equation for `it': it = foo `requires` bar
This can be avoided by specifying the result type:
*Propellor.Types> (foo `requires` bar) :: Property HasInfo
property "foo"
But then when multiple `requires` are given, the result type has to be
given each time:
*Propellor.Types> (foo `requires` bar `requires` bar) :: Property HasInfo
<interactive>:22:6:
No instance for (Requires (Property HasInfo) (Property HasInfo) x0)
arising from a use of `requires'
The type variable `x0' is ambiguous
Possible fix: add a type signature that fixes these type variable(s)
Note: there is a potential instance available:
instance Requires
(Property HasInfo) (Property HasInfo) (Property HasInfo)
-- Defined at Propellor/Types.hs:167:10
Possible fix:
add an instance declaration for
(Requires (Property HasInfo) (Property HasInfo) x0)
In the first argument of `requires', namely `foo `requires` bar'
In the expression:
(foo `requires` bar `requires` bar) :: Property HasInfo
In an equation for `it':
it = (foo `requires` bar `requires` bar) :: Property HasInfo
<interactive>:22:21:
No instance for (Requires x0 (Property HasInfo) (Property HasInfo))
arising from a use of `requires'
The type variable `x0' is ambiguous
Possible fix: add a type signature that fixes these type variable(s)
Note: there are several potential instances:
instance Requires
(Property NoInfo) (Property HasInfo) (Property HasInfo)
-- Defined at Propellor/Types.hs:175:10
instance Requires
(Property HasInfo) (Property HasInfo) (Property HasInfo)
-- Defined at Propellor/Types.hs:167:10
Possible fix:
add an instance declaration for
(Requires x0 (Property HasInfo) (Property HasInfo))
In the expression:
(foo `requires` bar `requires` bar) :: Property HasInfo
In an equation for `it':
it = (foo `requires` bar `requires` bar) :: Property HasInfo
*Propellor.Types> (((foo `requires` bar) :: Property HasInfo) `requires` bar) :: Property HasInfo
property "foo"
Yuggh!
Properties now form a tree, instead of the flat list used before.
This simplifies propigation of Info from the Properties used inside a
container to the outer host; the Property that docks the container on the
host can just have as child properties all the inner Properties, and their
Info can then be gathered recursively. (Although in practice it still needs
to be filtered, since not all Info should propigate out of a container.)
Note that there is no change to how Properties are actually satisfied.
Just because a Property lists some child properties, this does not mean
they always have their propertySatisfy actions run. It's still up to the
parent property to run those actions.
That's necessary so that a container's properties can be satisfied inside
it, not outside. It also allows property combinators to
add the combined Properties to their childProperties list, even if,
like onChange, they don't always run the child properties at all.
Testing: I tested that the exact same Info is calculated before and after
this change, for every Host in my config file.
Joey Hess