{-# LANGUAGE FlexibleContexts #-}

module Strategies where

import Control.Monad

import Base
import Representations


-- --------------------------------------
-- Strategies
-- --------------------------------------
-- Same monad to that in the SYB3 paper
data S m a = S a (m a)

instance MonadPlus m => Monad (S m) where
  return x = S x mzero
  (S x xs) >>= k = 
    S r (rs1 `mplus` rs2)
    where 
      S r rs1 = k x
      rs2     = 
        do x <- xs
           let S r _ = k x
           return r

-- Apply f to one of the immediate children of x
one :: (GMap (PF a), PFView a, Functor m, MonadPlus m) => (a -> m a) -> a -> m a
one f x = fmap to rs
  where 
    S _ rs = fmapM try (from x)
    try x = S x (f x)

-- Apply f once to one of the immediate children of x
once :: (GMap (PF a), PFView a, Functor m, MonadPlus m) => (a -> m a) -> a -> m a
once f x = f x `mplus` one (once f) x