Add (another) missing file

yesod-core/Yesod/Internal/Cache.hs

@@ -0,0 +1,38 @@
+{-# LANGUAGE GeneralizedNewtypeDeriving #-}
+{-# LANGUAGE TemplateHaskell #-}
+module Yesod.Internal.Cache
+    ( Cache
+    , CacheKey
+    , mkCacheKey
+    , lookup
+    , insert
+    , delete
+    ) where
+
+import Prelude hiding (lookup)
+import qualified Data.IntMap as Map
+import Language.Haskell.TH.Syntax (Q, Exp, runIO, Exp (LitE), Lit (IntegerL))
+import Language.Haskell.TH (appE)
+import Data.Unique (hashUnique, newUnique)
+import GHC.Exts (Any)
+import Unsafe.Coerce (unsafeCoerce)
+import Data.Monoid (Monoid)
+import Control.Applicative ((<$>))
+
+newtype Cache = Cache (Map.IntMap Any)
+    deriving Monoid
+
+newtype CacheKey a = CacheKey Int
+
+-- | Generate a new 'CacheKey'. Be sure to give a full type signature.
+mkCacheKey :: Q Exp
+mkCacheKey = [|CacheKey|] `appE` (LitE . IntegerL . fromIntegral . hashUnique <$> runIO newUnique)
+
+lookup :: CacheKey a -> Cache -> Maybe a
+lookup (CacheKey i) (Cache m) = unsafeCoerce <$> Map.lookup i m
+
+insert :: CacheKey a -> a -> Cache -> Cache
+insert (CacheKey k) v (Cache m) = Cache (Map.insert k (unsafeCoerce v) m)
+
+delete :: CacheKey a -> Cache -> Cache
+delete (CacheKey k) (Cache m) = Cache (Map.delete k m)