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!

1 files changed, 6 insertions, 6 deletions

diff --git a/src/Propellor/Info.hs b/src/Propellor/Info.hs
index 6d85cb74..1d8e7ab2 100644
--- a/src/Propellor/Info.hs
+++ b/src/Propellor/Info.hs
@@ -12,13 +12,13 @@ import Data.Maybe
 import Data.Monoid
 import Control.Applicative
 
-pureInfoProperty :: Desc -> Info -> Property 
+pureInfoProperty :: Desc -> Info -> Property HasInfo
 pureInfoProperty desc i = mkProperty ("has " ++ desc) (return NoChange) i mempty
 
 askInfo :: (Info -> Val a) -> Propellor (Maybe a)
 askInfo f = asks (fromVal . f . hostInfo)
 
-os :: System -> Property
+os :: System -> Property HasInfo
 os system = pureInfoProperty ("Operating " ++ show system) $
 	mempty { _os = Val system }
 
@@ -34,11 +34,11 @@ getOS = askInfo _os
 -- When propellor --spin is used to deploy a host, it checks
 -- if the host's IP Property matches the DNS. If the DNS is missing or
 -- out of date, the host will instead be contacted directly by IP address.
-ipv4 :: String -> Property
+ipv4 :: String -> Property HasInfo
 ipv4 = addDNS . Address . IPv4
 
 -- | Indidate that a host has an AAAA record in the DNS.
-ipv6 :: String -> Property
+ipv6 :: String -> Property HasInfo
 ipv6 = addDNS . Address . IPv6
 
 -- | Indicates another name for the host in the DNS.
@@ -47,7 +47,7 @@ ipv6 = addDNS . Address . IPv6
 -- to use their address, rather than using a CNAME. This avoids various
 -- problems with CNAMEs, and also means that when multiple hosts have the
 -- same alias, a DNS round-robin is automatically set up.
-alias :: Domain -> Property
+alias :: Domain -> Property HasInfo
 alias d = pureInfoProperty ("alias " ++ d) $ mempty
 	{ _aliases = S.singleton d
 	-- A CNAME is added here, but the DNS setup code converts it to an
@@ -55,7 +55,7 @@ alias d = pureInfoProperty ("alias " ++ d) $ mempty
 	, _dns = S.singleton $ CNAME $ AbsDomain d
 	} 
 
-addDNS :: Record -> Property
+addDNS :: Record -> Property HasInfo
 addDNS r = pureInfoProperty (rdesc r) $ mempty { _dns = S.singleton r }
   where
 	rdesc (CNAME d) = unwords ["alias", ddesc d]