Fix warnings for -XStarIsType (#149)

This is backwards compatible for all GHC >= 8.0

Author: jvanbruegge

Haxl/Core/DataSource.hs

@@ -57,6 +57,7 @@ import Control.Exception
 import Control.Monad
 import Data.Hashable
 import Data.Text (Text)
+import Data.Kind (Type)
 import Data.Typeable
 
 import Haxl.Core.Exception
@@ -109,7 +110,7 @@ class (DataSourceName req, StateKey req, ShowP req) => DataSource u req where
   classifyFailure :: u -> req a -> SomeException -> FailureClassification
   classifyFailure _ _ _ = StandardFailure
 
-class DataSourceName (req :: * -> *) where
+class DataSourceName (req :: Type -> Type) where
   -- | The name of this 'DataSource', used in tracing and stats. Must
   -- take a dummy request.
   dataSourceName :: Proxy req -> Text
@@ -130,7 +131,7 @@ type Request req a =
   )
 
 -- | Hints to the scheduler about this data source
-data SchedulerHint (req :: * -> *)
+data SchedulerHint (req :: Type -> Type)
   = TryToBatch
     -- ^ Hold data-source requests while we execute as much as we can, so
     -- that we can hopefully collect more requests to batch.

Haxl/Core/Fetch.hs

@@ -48,6 +48,7 @@ import Data.Monoid
 import Data.Proxy
 import Data.Typeable
 import Data.Text (Text)
+import Data.Kind (Type)
 import qualified Data.Text as Text
 import Text.Printf
 #ifdef PROFILING
@@ -316,7 +317,7 @@ dataFetchWithInsert showFn insertFn req =
 -- allows a transparent run afterwards. Without this, the test would try to
 -- call the datasource during testing and that would be an exception.
 uncachedRequest
- :: forall a u w (r :: * -> *). (DataSource u r, Request r a)
+ :: forall a u w (r :: Type -> Type). (DataSource u r, Request r a)
  => r a -> GenHaxl u w a
 uncachedRequest req = do
   flg <- env flags
@@ -485,7 +486,7 @@ performFetches env@Env{flags=f, statsRef=sref, statsBatchIdRef=sbref} jobs = do
 
 data FetchToDo where
   FetchToDo
-    :: forall (req :: * -> *). (DataSourceName req, Typeable req)
+    :: forall (req :: Type -> Type). (DataSourceName req, Typeable req)
     => [BlockedFetch req] -> PerformFetch req -> FetchToDo
 
 -- Catch exceptions arising from the data source and stuff them into

Haxl/Core/RequestStore.hs

@@ -44,6 +44,7 @@ import qualified Data.HashMap.Strict as HashMap
 import qualified Data.Map.Strict as Map
 import Data.Proxy
 import Data.Text (Text)
+import Data.Kind (Type)
 import Data.Typeable
 import Unsafe.Coerce
 
@@ -110,23 +111,23 @@ emptyReqCounts :: ReqCountMap
 emptyReqCounts = ReqCountMap Map.empty
 
 addToCountMap
-  :: forall (r :: * -> *). (DataSourceName r, Typeable r)
+  :: forall (r :: Type -> Type). (DataSourceName r, Typeable r)
   => Proxy r
   -> Int -- type and number of requests
   -> ReqCountMap
   -> ReqCountMap
 addToCountMap = updateCountMap (+)
 
 subFromCountMap
-  :: forall (r :: * -> *). (DataSourceName r, Typeable r)
+  :: forall (r :: Type -> Type). (DataSourceName r, Typeable r)
   => Proxy r
   -> Int -- type and number of requests
   -> ReqCountMap
   -> ReqCountMap
 subFromCountMap = updateCountMap (-)
 
 updateCountMap
-  :: forall (r :: * -> *). (DataSourceName r, Typeable r)
+  :: forall (r :: Type -> Type). (DataSourceName r, Typeable r)
   => (Int -> Int -> Int)
   -> Proxy r
   -> Int -- type and number of requests

Haxl/Core/StateStore.hs

@@ -23,6 +23,7 @@ module Haxl.Core.StateStore
   ) where
 
 import Data.Map (Map)
+import Data.Kind (Type)
 import qualified Data.Map.Strict as Map
 #if __GLASGOW_HASKELL__ < 804
 import Data.Monoid
@@ -34,7 +35,7 @@ import Unsafe.Coerce
 -- instance of 'StateKey' can store and retrieve information from a
 -- 'StateStore'.
 --
-class Typeable f => StateKey (f :: * -> *) where
+class Typeable f => StateKey (f :: Type -> Type) where
   data State f
 
   -- | We default this to typeOf1, but if f is itself a complex type that is