{-----------------------------------------------------------------------------
    reactive-banana
------------------------------------------------------------------------------}
module Tidings (
    -- * Synopsis
    -- The 'Tidings' data type for composing user events.
    --
    -- See <http://apfelmus.nfshost.com/blog/2012/03/29-frp-three-principles-bidirectional-gui.html>
    -- for more information.
    
    -- * Documentation
    Tidings, tidings, facts, rumors,
    ) where

import Reactive.Banana.Combinators

-- | Data type representing a behavior 'facts'
-- and suggestions to change it 'rumors'.
data Tidings t a = T { facts :: Behavior t a, rumors :: Event t a }

-- | Smart constructor. Combine facts and rumors into 'Tidings'.
tidings :: Behavior t a -> Event t a -> Tidings t a
tidings b e = T b (calm e)

instance Functor (Tidings t) where
    fmap f (T b e) = T (fmap f b) (fmap f e)

-- | The applicative instance combines 'rumors'
-- and uses 'facts' when some of the 'rumors' are not available.
instance Applicative (Tidings t) where
    pure x  = T (pure x) never
    f <*> x = uncurry ($) <$> pair f x

pair :: Tidings t a -> Tidings t b -> Tidings t (a,b)
pair (T bx ex) (T by ey) = T b e
    where
    b = (,) <$> bx <*> by
    x = flip (,) <$> by <@> ex
    y = (,) <$> bx <@> ey
    e = unionWith (\(x,_) (_,y) -> (x,y)) x y