From 45c94ffdd79b4f8134ef651b08fedb57b37448e6 Mon Sep 17 00:00:00 2001
From: Joey Hess <joeyh@joeyh.name>
Date: Sat, 24 Jan 2015 16:54:49 -0400
Subject: moving to using the GADT

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!
---
 src/Propellor/Engine.hs | 9 ++++-----
 1 file changed, 4 insertions(+), 5 deletions(-)

(limited to 'src/Propellor/Engine.hs')

diff --git a/src/Propellor/Engine.hs b/src/Propellor/Engine.hs
index ddc22305..552b910c 100644
--- a/src/Propellor/Engine.hs
+++ b/src/Propellor/Engine.hs
@@ -45,7 +45,7 @@ mainProperties host = do
 		FailedChange -> exitWith (ExitFailure 1)
 		_ -> exitWith ExitSuccess
   where
-	ps = hostProperties host
+	ps = map ignoreInfo $ hostProperties host
 
 -- | Runs a Propellor action with the specified host.
 --
@@ -66,13 +66,12 @@ runEndAction host res (EndAction desc a) = actionMessageOn (hostName host) desc
 -- | For when code running in the Propellor monad needs to ensure a
 -- Property.
 --
--- Note that the Info of the Property is not propigated out, so it will
--- not be visible to propellor, unless you arrange for it to be propigated.
-ensureProperty :: Property -> Propellor Result
+-- This can only be used on a Property that has NoInfo.
+ensureProperty :: Property NoInfo -> Propellor Result
 ensureProperty = catchPropellor . propertySatisfy
 
 -- | Ensures a list of Properties, with a display of each as it runs.
-ensureProperties :: [Property] -> Propellor Result
+ensureProperties :: [Property NoInfo] -> Propellor Result
 ensureProperties ps = ensure ps NoChange
   where
 	ensure [] rs = return rs
-- 
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