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-rw-r--r--src/Propellor/EnsureProperty.hs41
-rw-r--r--src/Propellor/PropAccum.hs17
-rw-r--r--src/Propellor/Property/Aiccu.hs5
-rw-r--r--src/Propellor/Property/Atomic.hs12
-rw-r--r--src/Propellor/Property/Systemd.hs1
-rw-r--r--src/Propellor/Property/Tor.hs30
-rw-r--r--src/Propellor/Types.hs36
-rw-r--r--src/Propellor/Types/MetaTypes.hs148
8 files changed, 210 insertions, 80 deletions
diff --git a/src/Propellor/EnsureProperty.hs b/src/Propellor/EnsureProperty.hs
index ab624706..5ed0325f 100644
--- a/src/Propellor/EnsureProperty.hs
+++ b/src/Propellor/EnsureProperty.hs
@@ -8,7 +8,7 @@ module Propellor.EnsureProperty
( ensureProperty
, property'
, OuterMetaTypesWitness
- , Cannot_ensureProperty_WithInfo
+ , EnsurePropertyAllowed
) where
import Propellor.Types
@@ -16,6 +16,8 @@ import Propellor.Types.Core
import Propellor.Types.MetaTypes
import Propellor.Exception
+import GHC.TypeLits
+import Data.Type.Bool
import Data.Monoid
import Prelude
@@ -41,19 +43,40 @@ ensureProperty
-- -Wredundant-constraints is turned off because
-- this constraint appears redundant, but is actually
-- crucial.
- ( Cannot_ensureProperty_WithInfo inner ~ 'True
- , (Targets inner `NotSuperset` Targets outer) ~ 'CanCombine
- )
+ ( EnsurePropertyAllowed inner outer ~ 'True)
=> OuterMetaTypesWitness outer
-> Property (MetaTypes inner)
-> Propellor Result
ensureProperty _ = maybe (return NoChange) catchPropellor . getSatisfy
--- The name of this was chosen to make type errors a bit more understandable.
-type family Cannot_ensureProperty_WithInfo (l :: [a]) :: Bool where
- Cannot_ensureProperty_WithInfo '[] = 'True
- Cannot_ensureProperty_WithInfo (t ': ts) =
- Not (t `EqT` 'WithInfo) && Cannot_ensureProperty_WithInfo ts
+type family EnsurePropertyAllowed inner outer where
+ EnsurePropertyAllowed inner outer =
+ (EnsurePropertyNoInfo inner)
+ &&
+ (EnsurePropertyTargetOSMatches inner outer)
+
+type family EnsurePropertyNoInfo (l :: [a]) :: Bool where
+ EnsurePropertyNoInfo '[] = 'True
+ EnsurePropertyNoInfo (t ': ts) = If (Not (t `EqT` 'WithInfo))
+ (EnsurePropertyNoInfo ts)
+ (TypeError ('Text "Cannot use ensureProperty with a Property that HasInfo."))
+
+type family EnsurePropertyTargetOSMatches inner outer where
+ EnsurePropertyTargetOSMatches inner outer =
+ If (Targets outer `IsSubset` Targets inner)
+ 'True
+ (IfStuck (Targets outer)
+ (TypeError
+ ('Text "ensureProperty outer Property type is not able to be inferred here."
+ ':$$: 'Text "Consider adding a type annotation."
+ )
+ )
+ (TypeError
+ ('Text "ensureProperty inner Property is missing support for: "
+ ':$$: PrettyPrintMetaTypes (Difference (Targets outer) (Targets inner))
+ )
+ )
+ )
-- | Constructs a property, like `property`, but provides its
-- `OuterMetaTypesWitness`.
diff --git a/src/Propellor/PropAccum.hs b/src/Propellor/PropAccum.hs
index c60ced73..3fc83202 100644
--- a/src/Propellor/PropAccum.hs
+++ b/src/Propellor/PropAccum.hs
@@ -19,6 +19,7 @@ import Propellor.Types.MetaTypes
import Propellor.Types.Core
import Propellor.Property
+import GHC.TypeLits
import Data.Monoid
import Prelude
@@ -45,6 +46,16 @@ type family GetMetaTypes x where
GetMetaTypes (Property (MetaTypes t)) = MetaTypes t
GetMetaTypes (RevertableProperty (MetaTypes t) undo) = MetaTypes t
+-- When many properties are combined, ghc error message
+-- can include quite a lot of code, typically starting with
+-- `props and including all the properties up to and including the
+-- one that fails to combine. Point the user in the right direction.
+type family NoteFor symbol :: ErrorMessage where
+ NoteFor symbol =
+ 'Text "Probably the problem is with the last property added with "
+ ':<>: symbol
+ ':<>: 'Text " in the code excerpt below."
+
-- | Adds a property to a Props.
--
-- Can add Properties and RevertableProperties
@@ -55,7 +66,7 @@ type family GetMetaTypes x where
-- this constraint appears redundant, but is actually
-- crucial.
, MetaTypes y ~ GetMetaTypes p
- , CheckCombinable x y ~ 'CanCombine
+ , CheckCombinableNote x y (NoteFor ('Text "&")) ~ 'True
)
=> Props (MetaTypes x)
-> p
@@ -70,7 +81,7 @@ Props c & p = Props (c ++ [toChildProperty p])
-- this constraint appears redundant, but is actually
-- crucial.
, MetaTypes y ~ GetMetaTypes p
- , CheckCombinable x y ~ 'CanCombine
+ , CheckCombinableNote x y (NoteFor ('Text "&^")) ~ 'True
)
=> Props (MetaTypes x)
-> p
@@ -82,7 +93,7 @@ Props c &^ p = Props (toChildProperty p : c)
-- -Wredundant-constraints is turned off because
-- this constraint appears redundant, but is actually
-- crucial.
- :: (CheckCombinable x z ~ 'CanCombine)
+ :: (CheckCombinableNote x z (NoteFor ('Text "!")) ~ 'True)
=> Props (MetaTypes x)
-> RevertableProperty (MetaTypes y) (MetaTypes z)
-> Props (MetaTypes (Combine x z))
diff --git a/src/Propellor/Property/Aiccu.hs b/src/Propellor/Property/Aiccu.hs
index 1b28759c..8bf3f283 100644
--- a/src/Propellor/Property/Aiccu.hs
+++ b/src/Propellor/Property/Aiccu.hs
@@ -47,8 +47,7 @@ hasConfig :: TunnelId -> UserName -> Property (HasInfo + DebianLike)
hasConfig t u = prop `onChange` restarted
where
prop :: Property (HasInfo + UnixLike)
- prop = withSomePrivData [(Password (u++"/"++t)), (Password u)] (Context "aiccu") $
- property' "aiccu configured" . writeConfig
- writeConfig getpassword w = getpassword $ ensureProperty w . go
+ prop = withSomePrivData [(Password (u++"/"++t)), (Password u)] (Context "aiccu") $ \getpassword ->
+ property' "aiccu configured" $ \w -> getpassword $ ensureProperty w . go
go (Password u', p) = confPath `File.hasContentProtected` config u' t p
go (f, _) = error $ "Unexpected type of privdata: " ++ show f
diff --git a/src/Propellor/Property/Atomic.hs b/src/Propellor/Property/Atomic.hs
index 8519048b..2c7433f6 100644
--- a/src/Propellor/Property/Atomic.hs
+++ b/src/Propellor/Property/Atomic.hs
@@ -46,10 +46,8 @@ type CheckAtomicResourcePair a = AtomicResourcePair a -> Propellor (AtomicResour
-- inactiveAtomicResource, and if it was successful,
-- atomically activating that resource.
atomicUpdate
- -- Constriaints inherited from ensureProperty.
- :: ( Cannot_ensureProperty_WithInfo t ~ 'True
- , (Targets t `NotSuperset` Targets t) ~ 'CanCombine
- )
+ -- Constriaint inherited from ensureProperty.
+ :: (EnsurePropertyAllowed t t ~ 'True)
=> SingI t
=> AtomicResourcePair a
-> CheckAtomicResourcePair a
@@ -92,10 +90,8 @@ atomicUpdate rbase rcheck rswap mkp = property' d $ \w -> do
-- children: a symlink with the name of the directory itself, and two copies
-- of the directory, with names suffixed with ".1" and ".2"
atomicDirUpdate
- -- Constriaints inherited from ensureProperty.
- :: ( Cannot_ensureProperty_WithInfo t ~ 'True
- , (Targets t `NotSuperset` Targets t) ~ 'CanCombine
- )
+ -- Constriaint inherited from ensureProperty.
+ :: (EnsurePropertyAllowed t t ~ 'True)
=> SingI t
=> FilePath
-> (FilePath -> Property (MetaTypes t))
diff --git a/src/Propellor/Property/Systemd.hs b/src/Propellor/Property/Systemd.hs
index bfc0f9a5..5d597ac6 100644
--- a/src/Propellor/Property/Systemd.hs
+++ b/src/Propellor/Property/Systemd.hs
@@ -391,6 +391,7 @@ mungename = replace "/" "_"
containerCfg :: String -> RevertableProperty (HasInfo + Linux) (HasInfo + Linux)
containerCfg p = RevertableProperty (mk True) (mk False)
where
+ mk :: Bool -> Property (HasInfo + Linux)
mk b = tightenTargets $
pureInfoProperty desc $
mempty { _chrootCfg = SystemdNspawnCfg [(p', b)] }
diff --git a/src/Propellor/Property/Tor.hs b/src/Propellor/Property/Tor.hs
index 426d4209..862af983 100644
--- a/src/Propellor/Property/Tor.hs
+++ b/src/Propellor/Property/Tor.hs
@@ -172,20 +172,22 @@ hiddenServiceData hn context = combineProperties desc $ props
where
desc = unwords ["hidden service data available in", varLib </> hn]
installonion :: FilePath -> Property (HasInfo + DebianLike)
- installonion f = withPrivData (PrivFile $ varLib </> hn </> f) context $ \getcontent ->
- property' desc $ \w -> getcontent $ install w $ varLib </> hn </> f
- install w f privcontent = ifM (liftIO $ doesFileExist f)
- ( noChange
- , ensureProperty w $ propertyList desc $ toProps
- [ property desc $ makeChange $ do
- createDirectoryIfMissing True (takeDirectory f)
- writeFileProtected f (unlines (privDataLines privcontent))
- , File.mode (takeDirectory f) $ combineModes
- [ownerReadMode, ownerWriteMode, ownerExecuteMode]
- , File.ownerGroup (takeDirectory f) user (userGroup user)
- , File.ownerGroup f user (userGroup user)
- ]
- )
+ installonion basef =
+ let f = varLib </> hn </> basef
+ in withPrivData (PrivFile f) context $ \getcontent ->
+ property' desc $ \w -> getcontent $ \privcontent ->
+ ifM (liftIO $ doesFileExist f)
+ ( noChange
+ , ensureProperty w $ propertyList desc $ toProps
+ [ property desc $ makeChange $ do
+ createDirectoryIfMissing True (takeDirectory f)
+ writeFileProtected f (unlines (privDataLines privcontent))
+ , File.mode (takeDirectory f) $ combineModes
+ [ownerReadMode, ownerWriteMode, ownerExecuteMode]
+ , File.ownerGroup (takeDirectory f) user (userGroup user)
+ , File.ownerGroup f user (userGroup user)
+ ]
+ )
restarted :: Property DebianLike
restarted = Service.restarted "tor"
diff --git a/src/Propellor/Types.hs b/src/Propellor/Types.hs
index 7052bf92..026babf0 100644
--- a/src/Propellor/Types.hs
+++ b/src/Propellor/Types.hs
@@ -31,6 +31,7 @@ module Propellor.Types (
, HasInfo
, type (+)
, TightenTargets(..)
+ , TightenTargetsAllowed
-- * Combining and modifying properties
, Combines(..)
, CombinedType
@@ -44,6 +45,8 @@ module Propellor.Types (
, module Propellor.Types.ZFS
) where
+import GHC.TypeLits hiding (type (+))
+import Data.Type.Bool
import qualified Data.Semigroup as Sem
import Data.Monoid
import Control.Applicative
@@ -59,7 +62,7 @@ import Propellor.Types.MetaTypes
import Propellor.Types.ZFS
-- | The core data type of Propellor, this represents a property
--- that the system should have, with a descrition, and an action to ensure
+-- that the system should have, with a description, and an action to ensure
-- it has the property.
--
-- There are different types of properties that target different OS's,
@@ -185,17 +188,17 @@ class Combines x y where
-> y
-> CombinedType x y
-instance (CheckCombinable x y ~ 'CanCombine, SingI (Combine x y)) => Combines (Property (MetaTypes x)) (Property (MetaTypes y)) where
+instance (CheckCombinable x y ~ 'True, SingI (Combine x y)) => Combines (Property (MetaTypes x)) (Property (MetaTypes y)) where
combineWith f _ (Property _ d1 a1 i1 c1) (Property _ d2 a2 i2 c2) =
Property sing d1 (f a1 a2) i1 (ChildProperty d2 a2 i2 c2 : c1)
-instance (CheckCombinable x y ~ 'CanCombine, CheckCombinable x' y' ~ 'CanCombine, SingI (Combine x y), SingI (Combine x' y')) => Combines (RevertableProperty (MetaTypes x) (MetaTypes x')) (RevertableProperty (MetaTypes y) (MetaTypes y')) where
+instance (CheckCombinable x y ~ 'True, CheckCombinable x' y' ~ 'True, SingI (Combine x y), SingI (Combine x' y')) => Combines (RevertableProperty (MetaTypes x) (MetaTypes x')) (RevertableProperty (MetaTypes y) (MetaTypes y')) where
combineWith sf tf (RevertableProperty s1 t1) (RevertableProperty s2 t2) =
RevertableProperty
(combineWith sf tf s1 s2)
(combineWith tf sf t1 t2)
-instance (CheckCombinable x y ~ 'CanCombine, SingI (Combine x y)) => Combines (RevertableProperty (MetaTypes x) (MetaTypes x')) (Property (MetaTypes y)) where
+instance (CheckCombinable x y ~ 'True, SingI (Combine x y)) => Combines (RevertableProperty (MetaTypes x) (MetaTypes x')) (Property (MetaTypes y)) where
combineWith sf tf (RevertableProperty x _) y = combineWith sf tf x y
-instance (CheckCombinable x y ~ 'CanCombine, SingI (Combine x y)) => Combines (Property (MetaTypes x)) (RevertableProperty (MetaTypes y) (MetaTypes y')) where
+instance (CheckCombinable x y ~ 'True, SingI (Combine x y)) => Combines (Property (MetaTypes x)) (RevertableProperty (MetaTypes y) (MetaTypes y')) where
combineWith sf tf x (RevertableProperty y _) = combineWith sf tf x y
class TightenTargets p where
@@ -209,14 +212,31 @@ class TightenTargets p where
-- > upgraded = tightenTargets $ cmdProperty "apt-get" ["upgrade"]
tightenTargets
::
- -- Note that this uses PolyKinds
- ( (Targets untightened `NotSuperset` Targets tightened) ~ 'CanCombine
- , (NonTargets tightened `NotSuperset` NonTargets untightened) ~ 'CanCombine
+ ( TightenTargetsAllowed untightened tightened ~ 'True
, SingI tightened
)
=> p (MetaTypes untightened)
-> p (MetaTypes tightened)
+-- Note that this uses PolyKinds
+type family TightenTargetsAllowed untightened tightened where
+ TightenTargetsAllowed untightened tightened =
+ If (Targets tightened `IsSubset` Targets untightened
+ && NonTargets untightened `IsSubset` NonTargets tightened)
+ 'True
+ (IfStuck (Targets tightened)
+ (TypeError
+ ('Text "Unable to infer desired Property type in this use of tightenTargets."
+ ':$$: ('Text "Consider adding a type annotation.")
+ )
+ )
+ (TypeError
+ ('Text "This use of tightenTargets would widen, not narrow, adding: "
+ ':$$: PrettyPrintMetaTypes (Difference (Targets tightened) (Targets untightened))
+ )
+ )
+ )
+
instance TightenTargets Property where
tightenTargets (Property _ d a i c) = Property sing d a i c
diff --git a/src/Propellor/Types/MetaTypes.hs b/src/Propellor/Types/MetaTypes.hs
index 0c476e87..567f533a 100644
--- a/src/Propellor/Types/MetaTypes.hs
+++ b/src/Propellor/Types/MetaTypes.hs
@@ -1,4 +1,5 @@
{-# LANGUAGE TypeOperators, PolyKinds, DataKinds, TypeFamilies, UndecidableInstances, FlexibleInstances, GADTs #-}
+{-# LANGUAGE CPP #-}
module Propellor.Types.MetaTypes (
MetaType(..),
@@ -17,21 +18,33 @@ module Propellor.Types.MetaTypes (
IncludesInfo,
Targets,
NonTargets,
- NotSuperset,
+ PrettyPrintMetaTypes,
+ IsSubset,
Combine,
- CheckCombine(..),
CheckCombinable,
+ CheckCombinableNote,
type (&&),
Not,
EqT,
Union,
+ Intersect,
+ Difference,
+ IfStuck,
) where
import Propellor.Types.Singletons
import Propellor.Types.OS
+import GHC.TypeLits hiding (type (+))
import Data.Type.Bool
+#ifdef WITH_TYPE_ERRORS
+import Type.Errors
+#else
+type family IfStuck (expr :: k) (b :: k1) (c :: k1) :: k1 where
+ IfStuck expr b c = c
+#endif
+
data MetaType
= Targeting TargetOS -- ^ A target OS of a Property
| WithInfo -- ^ Indicates that a Property has associated Info
@@ -113,41 +126,69 @@ type family Combine (list1 :: [a]) (list2 :: [a]) :: [a] where
(Targets list1 `Intersect` Targets list2)
)
--- | Checks if two MetaTypes lists can be safely combined.
---
--- This should be used anywhere Combine is used, as an additional
--- constraint. For example:
---
--- > foo :: (CheckCombinable x y ~ 'CanCombine) => x -> y -> Combine x y
-type family CheckCombinable (list1 :: [a]) (list2 :: [a]) :: CheckCombine where
- -- As a special case, if either list is empty, let it be combined
- -- with the other. This relies on MetaTypes list always containing
- -- at least one target, so can only happen if there's already been
- -- a type error. This special case lets the type checker show only
- -- the original type error, and not an extra error due to a later
- -- CheckCombinable constraint.
- CheckCombinable '[] list2 = 'CanCombine
- CheckCombinable list1 '[] = 'CanCombine
- CheckCombinable (l1 ': list1) (l2 ': list2) =
- CheckCombinable' (Combine (l1 ': list1) (l2 ': list2))
-type family CheckCombinable' (combinedlist :: [a]) :: CheckCombine where
- CheckCombinable' '[] = 'CannotCombineTargets
- CheckCombinable' (a ': rest)
- = If (IsTarget a)
- 'CanCombine
- (CheckCombinable' rest)
-
-data CheckCombine = CannotCombineTargets | CanCombine
+-- | Checks if two MetaTypes lists can be safly combined;
+-- eg they have at least one Target in common.
+type family IsCombinable (list1 :: [a]) (list2 :: [a]) :: Bool where
+ -- As a special case, if either list is empty or only WithInfo,
+ -- let it be combined with the other. This relies on MetaTypes
+ -- list always containing at least one Target, so can only happen
+ -- if there's already been a type error. This special case lets the
+ -- type checker show only the original type error, and not
+ -- subsequent errors due to later CheckCombinable constraints.
+ IsCombinable '[] list2 = 'True
+ IsCombinable list1 '[] = 'True
+ IsCombinable ('WithInfo ': list1) list2 = IsCombinable list1 list2
+ IsCombinable list1 ('WithInfo ': list2) = IsCombinable list1 list2
+ IsCombinable list1 list2 =
+ Not (Null (Combine (Targets list1) (Targets list2)))
--- | Every item in the subset must be in the superset.
+-- | This (or CheckCombinableNote) should be used anywhere Combine is used,
+-- as an additional constraint. For example:
--
--- The name of this was chosen to make type errors more understandable.
-type family NotSuperset (superset :: [a]) (subset :: [a]) :: CheckCombine where
- NotSuperset superset '[] = 'CanCombine
- NotSuperset superset (s ': rest) =
- If (Elem s superset)
- (NotSuperset superset rest)
- 'CannotCombineTargets
+-- > foo :: (CheckCombinable x y ~ 'True) => x -> y -> Combine x y
+type family CheckCombinable (list1 :: [a]) (list2 :: [a]) :: Bool where
+ CheckCombinable list1 list2 =
+ If (IsCombinable list1 list2)
+ 'True
+ (CannotCombine list1 list2 'Nothing)
+
+-- | Allows providing an additional note.
+type family CheckCombinableNote (list1 :: [a]) (list2 :: [a]) (note :: ErrorMessage) :: Bool where
+ CheckCombinableNote list1 list2 note =
+ If (IsCombinable list1 list2)
+ 'True
+ (CannotCombine list1 list2
+ ('Just ('Text "(" ':<>: note ':<>: 'Text ")"))
+ )
+
+-- Checking IfStuck is to avoid massive and useless error message leaking
+-- type families from this module.
+type family CannotCombine (list1 :: [a]) (list2 :: [a]) (note :: Maybe ErrorMessage) :: Bool where
+ CannotCombine list1 list2 note =
+ IfStuck list1
+ (IfStuck list2
+ (TypeError (CannotCombineMessage UnknownType UnknownType UnknownTypeNote))
+ (TypeError (CannotCombineMessage UnknownType (PrettyPrintMetaTypes list2) UnknownTypeNote))
+ )
+ (IfStuck list2
+ (TypeError (CannotCombineMessage (PrettyPrintMetaTypes list1) UnknownType UnknownTypeNote))
+ (TypeError (CannotCombineMessage (PrettyPrintMetaTypes list1) (PrettyPrintMetaTypes list2) note))
+ )
+
+type family UnknownType :: ErrorMessage where
+ UnknownType = 'Text "<unknown>"
+
+type family UnknownTypeNote :: Maybe ErrorMessage where
+ UnknownTypeNote = 'Just ('Text "(Property <unknown> is often due to a partially applied Property constructor, or due to passing the wrong type to a Property constructor.)")
+
+type family CannotCombineMessage (a :: ErrorMessage) (b :: ErrorMessage) (note :: Maybe ErrorMessage) :: ErrorMessage where
+ CannotCombineMessage a b ('Just note) =
+ CannotCombineMessage a b 'Nothing
+ ':$$: note
+ CannotCombineMessage a b 'Nothing =
+ 'Text "Cannot combine Properties:"
+ ':$$: ('Text " Property " ':<>: a)
+ ':$$: ('Text " Property " ':<>: b)
type family IsTarget (a :: t) :: Bool where
IsTarget ('Targeting a) = 'True
@@ -167,6 +208,21 @@ type family NonTargets (l :: [a]) :: [a] where
(NonTargets xs)
(x ': NonTargets xs)
+-- | Pretty-prints a list of MetaTypes for display in a type error message.
+type family PrettyPrintMetaTypes (l :: [MetaType]) :: ErrorMessage where
+ PrettyPrintMetaTypes '[] = 'Text "<none>"
+ PrettyPrintMetaTypes (t ': '[]) = PrettyPrintMetaType t
+ PrettyPrintMetaTypes (t ': ts) =
+ PrettyPrintMetaType t ':<>: 'Text " + " ':<>: PrettyPrintMetaTypes ts
+
+type family PrettyPrintMetaType t :: ErrorMessage where
+ PrettyPrintMetaType 'WithInfo = 'ShowType HasInfo
+ PrettyPrintMetaType ('Targeting 'OSDebian) = 'ShowType Debian
+ PrettyPrintMetaType ('Targeting 'OSBuntish) = 'ShowType Buntish
+ PrettyPrintMetaType ('Targeting 'OSFreeBSD) = 'ShowType FreeBSD
+ PrettyPrintMetaType ('Targeting 'OSArchLinux) = 'ShowType ArchLinux
+ PrettyPrintMetaType ('Targeting t) = 'ShowType t
+
-- | Type level elem
type family Elem (a :: t) (list :: [t]) :: Bool where
Elem a '[] = 'False
@@ -188,6 +244,28 @@ type family Intersect (list1 :: [a]) (list2 :: [a]) :: [a] where
(a ': Intersect rest list2)
(Intersect rest list2)
+-- | Type level difference. Items that are in the first list, but not in
+-- the second.
+type family Difference (list1 :: [a]) (list2 :: [a]) :: [a] where
+ Difference '[] list2 = '[]
+ Difference (a ': rest) list2 =
+ If (Elem a list2)
+ (Difference rest list2)
+ (a ': Difference rest list2)
+
+-- | Every item in the subset must be in the superset.
+type family IsSubset (subset :: [a]) (superset :: [a]) :: Bool where
+ IsSubset '[] superset = 'True
+ IsSubset (s ': rest) superset =
+ If (Elem s superset)
+ (IsSubset rest superset)
+ 'False
+
+-- | Type level null.
+type family Null (list :: [a]) :: Bool where
+ Null '[] = 'True
+ Null l = 'False
+
-- | Type level equality of metatypes.
type family EqT (a :: MetaType) (b :: MetaType) where
EqT a a = 'True