    {-# LANGUAGE GeneralizedNewtypeDeriving, MultiParamTypeClasses #-}
    {-# OPTIONS_GHC -funbox-strict-fields #-}

    -- |
    -- Module      : Criterion.Monad.Internal
    -- Copyright   : (c) 2009 Neil Brown
    --
    -- License     : BSD-style
    -- Maintainer  : bos@serpentine.com
    -- Stability   : experimental
    -- Portability : GHC
    --
    -- The environment in which most criterion code executes.
    module Criterion.Monad.Internal
        (
          Criterion(..)
        , Crit(..)
        ) where

    import Control.Monad.Catch (MonadThrow, MonadCatch, MonadMask)
    import qualified Control.Monad.Fail as Fail (MonadFail(..))
    import Control.Monad.Reader (MonadReader(..), ReaderT)
    import Control.Monad.Trans (MonadIO)
    import Control.Monad.Trans.Instances ()
    import Criterion.Types (Config)
    import Data.IORef (IORef)
    import Prelude ()
    import Prelude.Compat
    import System.Random.MWC (GenIO)

    data Crit = Crit {
        config   :: !Config
      , gen      :: !(IORef (Maybe GenIO))
      }

    -- | The monad in which most criterion code executes.
    newtype Criterion a = Criterion {
          runCriterion :: ReaderT Crit IO a
        } deriving ( Functor, Applicative, Monad, Fail.MonadFail, MonadIO
                   , MonadThrow, MonadCatch, MonadMask )

    instance MonadReader Config Criterion where
        ask     = config `fmap` Criterion ask
        local f = Criterion . local fconfig . runCriterion
          where fconfig c = c { config = f (config c) }