Holding initial value

[ivanperez-keera]

While playing around, I wondered whether it's possible in Copilot to hold the initial value of a stream, and came up with the following example:

-- | Holding initial values
{-# LANGUAGE RebindableSyntax #-}

module Main where

import Copilot.Compile.C99
import Data.Default
import Language.Copilot

instance Default Bool where
  def = False

-- | A stream that has the initial value of another stream.
initial :: (Typed a, Default a) => Stream a -> Stream a
initial s = g
  where
    g = mux itp s ([def] ++ g)

-- | Initial time point
itp :: Stream Bool
itp = [True] ++ false

-- * Example

main :: IO ()
main = interpret 4 spec

-- | Show initial value of ext for all time
spec :: Spec
spec = trigger "ext1" true [arg $ ext1]

-- | Initial value of ext for all time
ext1 :: Stream Bool
ext1 = initial ext

ext :: Stream Bool
ext = extern "ext" (Just [False, True, True, False])

This is related to but not the same as stuttering (#691).