{-# LANGUAGE TemplateHaskell #-}
{-# LANGUAGE MultiParamTypeClasses #-}
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE FunctionalDependencies #-}

module Events.Types
       ( TimeRange(..), rangeStart, rangeDuration
       , Event(..), payload, occursWithin
       , EvalCtx(..), ctxVars, ctxEvents
       , Eval, evaluate
       , module Data.Aeson
       , module Data.Time.Clock
       , module Data.Default.Class
       ) where

import Control.Lens.TH
  
import Data.Aeson (Object)

import Data.Time.Clock

import Control.Monad.State.Lazy
import ListT (ListT)
import qualified ListT

import Data.Default.Class

import Data.Monoid
import Control.Monad.Fix
import Control.Lens
import Data.Maybe

data TimeRange = TimeRange
                 { _rangeStart :: UTCTime
                 , _rangeDuration :: NominalDiffTime
                 }
makeLenses ''TimeRange

data Event = Event
             { _payload :: Object
             , _occursWithin :: TimeRange -> Bool
             }
makeLenses ''Event

data EvalCtx = EvalCtx
               { _ctxVars :: Object
               , _ctxEvents :: [Event]
               }
makeLenses ''EvalCtx

instance Default EvalCtx where
  def = EvalCtx
        { _ctxVars = mempty
        , _ctxEvents = mempty
        }

type Eval m a = ListT (StateT EvalCtx m) a

evaluate :: MonadFix m => Eval m (Maybe Event) -> m [Event]
evaluate x = catMaybes <$> mfix x'
  where
    x' = evalStateT (ListT.toList x) . flip (set ctxEvents) def . catMaybes