It didn't do what I thought it did with a RevertableProperty; it always
returned Nothing because even if the input properties to <!> are NoInfo, it
casts them to HasInfo.
Even if it had worked, it lost type safety. Better to export the
Property NoInfo that is used in a RevertableProperty, so it can be used
directly.
The old one caused the actions to run in the right order, but with the
wrong description.
This problem was found by comparing the [Host] between this branch and
current joeyconfig, and printing out their properties, info, and also their
list of child properties.
The only other difference found is that onChange orders the child property
list differently. That does not have any real effect and would be difficult
to change, so I've left it as-is.
* Property has been converted to a GADT, and will be Property NoInfo
or Property HasInfo.
This was done to make sure that ensureProperty is only used on
properties that do not have Info.
Transition guide:
- Change all "Property" to "Property NoInfo" or "Property WithInfo"
(The compiler can tell you if you got it wrong!)
- To construct a RevertableProperty, it is useful to use the new
(<!>) operator
- Constructing a list of properties can be problimatic, since
Property NoInto and Property WithInfo are different types and cannot
appear in the same list. To deal with this, "props" has been added,
and can built up a list of properties of different types,
using the same (&) and (!) operators that are used to build
up a host's properties.
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!
Joey Hess