Add MonadFake class for lifting Fakes

Following on from the 'FakeT' transformer, it makes sense to be able to
"lift" the default 'Fake' values into the monad stack, in order to allow
them to actually share the transformer cache and generator. This is done
by the 'MonadFake' class, which comes with an instance for 'FakeT', as
well as some of the other transformers defined in the `transformers`
package.

fakedata.cabal

@@ -273,6 +273,7 @@ library
       Faker.Cannabis
       Faker.Chiquito
       Faker.ChuckNorris
+      Faker.Class
       Faker.Code
       Faker.Coffee
       Faker.Coin
@@ -609,6 +610,7 @@ library
     , template-haskell
     , text
     , time
+    , transformers
     , unordered-containers
     , vector
     , yaml

src/Faker/Class.hs

@@ -0,0 +1,33 @@
+module Faker.Class
+  ( MonadFake(..)
+  ) where
+
+import Faker (FakeT(..), Fake)
+
+import Control.Monad.IO.Class
+import Control.Monad.Trans.Class (MonadTrans(lift))
+import Control.Monad.Trans.Except (ExceptT)
+import Control.Monad.Trans.Identity (IdentityT)
+import Control.Monad.Trans.Maybe (MaybeT)
+import Control.Monad.Trans.Reader (ReaderT)
+import Control.Monad.Trans.State (StateT)
+import Control.Monad.Trans.Writer (WriterT)
+
+class Monad m => MonadFake m where
+  liftFake :: Fake a -> m a
+
+instance MonadIO m => MonadFake (FakeT m) where
+  liftFake (Fake f) = FakeT (liftIO . f)
+
+instance MonadFake m => MonadFake (ReaderT r m) where
+  liftFake = lift . liftFake
+instance (Monoid w, MonadFake m) => MonadFake (WriterT w m) where
+  liftFake = lift . liftFake
+instance MonadFake m => MonadFake (StateT s m) where
+  liftFake = lift . liftFake
+instance MonadFake m => MonadFake (IdentityT m) where
+  liftFake = lift . liftFake
+instance MonadFake m => MonadFake (ExceptT e m) where
+  liftFake = lift . liftFake
+instance MonadFake m => MonadFake (MaybeT m) where
+  liftFake = lift . liftFake