{-# LANGUAGE ScopedTypeVariables #-}
{-# LANGUAGE TypeApplications    #-}
{-# LANGUAGE TypeFamilies        #-}

module Mutual where

import           Datafix
import           Numeric.Natural

-- | A 'DataFlowProblem' with two nodes, mutually depending on another, like
--
-- @
--    a = b + 1
--    b = min a 10
-- @
--
-- After a few bounces, this will reach a stable state where the first node
-- has value 11 and the other has value 10.
mutualRecursiveProblem :: forall m . (MonadDependency m, Domain m ~ Natural) => DataFlowProblem m
mutualRecursiveProblem = DFP transfer (const (eqChangeDetector @(Domain m)))
  where
    transfer :: Node -> LiftedFunc Natural m
    transfer (Node 0) = do
      b <- dependOn @m (Node 1)
      return (b + 1)
    transfer (Node 1) = do
      a <- dependOn @m (Node 0)
      return (min 10 a) -- So the overall fixpoint of this is 10
    transfer (Node _) = error "Invalid node"