-- |
-- Module      :  FRP.Yampa.Loop
-- Copyright   :  (c) Ivan Perez, 2014-2022
--                (c) George Giorgidze, 2007-2012
--                (c) Henrik Nilsson, 2005-2006
--                (c) Antony Courtney and Henrik Nilsson, Yale University, 2003-2004
-- License     :  BSD-style (see the LICENSE file in the distribution)
--
-- Maintainer  :  ivan.perez@keera.co.uk
-- Stability   :  provisional
--
-- Portability :  non-portable -GHC extensions-
--
-- Well-initialised loops
module FRP.Yampa.Loop
    (
      -- * Loops with guaranteed well-defined feedback
      loopPre
    , loopIntegral
    )
  where

import Control.Arrow
import Data.VectorSpace

import FRP.Yampa.Delays
import FRP.Yampa.Integration
import FRP.Yampa.InternalCore (SF)

-- * Loops with guaranteed well-defined feedback

-- | Loop with an initial value for the signal being fed back.
loopPre :: c -> SF (a,c) (b,c) -> SF a b
loopPre c_init sf = loop (second (iPre c_init) >>> sf)

-- | Loop by integrating the second value in the pair and feeding the
-- result back. Because the integral at time 0 is zero, this is always
-- well defined.
loopIntegral :: (Fractional s, VectorSpace c s) => SF (a,c) (b,c) -> SF a b
loopIntegral sf = loop (second integral >>> sf)