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-- | Monad transformer stack used by the @GraphQL@ resolvers.
module Language.GraphQL.Trans
( ActionT(..)
, Context(Context)
) where
import Control.Applicative (Alternative(..))
import Control.Monad (MonadPlus(..))
import Control.Monad.IO.Class (MonadIO(..))
import Control.Monad.Trans.Class (MonadTrans(..))
import Control.Monad.Trans.Except (ExceptT)
import Control.Monad.Trans.Reader (ReaderT)
import Data.HashMap.Strict (HashMap)
import Data.Text (Text)
import Language.GraphQL.AST.Core (Name, Value)
-- | Resolution context holds resolver arguments.
newtype Context = Context (HashMap Name Value)
-- | Monad transformer stack used by the resolvers to provide error handling
-- and resolution context (resolver arguments).
newtype ActionT m a = ActionT
{ runActionT :: ExceptT Text (ReaderT Context m) a
}
instance Functor m => Functor (ActionT m) where
fmap f = ActionT . fmap f . runActionT
instance Monad m => Applicative (ActionT m) where
pure = ActionT . pure
(ActionT f) <*> (ActionT x) = ActionT $ f <*> x
instance Monad m => Monad (ActionT m) where
return = pure
(ActionT action) >>= f = ActionT $ action >>= runActionT . f
instance MonadTrans ActionT where
lift = ActionT . lift . lift
instance MonadIO m => MonadIO (ActionT m) where
liftIO = lift . liftIO
instance Monad m => Alternative (ActionT m) where
empty = ActionT empty
(ActionT x) <|> (ActionT y) = ActionT $ x <|> y
instance Monad m => MonadPlus (ActionT m) where
mzero = empty
mplus = (<|>)
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