monad-time-0.2: src/Control/Monad/Time.hs

{-# OPTIONS_GHC -fno-warn-deprecated-flags #-}
{-# LANGUAGE FlexibleContexts, FlexibleInstances, OverlappingInstances
  , UndecidableInstances #-}
module Control.Monad.Time (MonadTime(..)) where

import Control.Monad.Trans
import Data.Time

-- | Class of monads which carry the notion of the current time.
class Monad m => MonadTime m where
  currentTime :: m UTCTime

-- | Base instance for IO.
instance MonadTime IO where
  currentTime = getCurrentTime

-- | Generic, overlapping instance.
instance (
    MonadTime m
  , MonadTrans t
  , Monad (t m)
  ) => MonadTime (t m) where
    currentTime = lift currentTime