amb_like.hs

{-# LANGUAGE GeneralisedNewtypeDeriving #-}
{-# LANGUAGE ScopedTypeVariables #-}

module Amb where

import Control.Monad.Reader
import Control.Monad.Cont
import Data.Maybe (isJust)
import Data.List (find)


require :: Eq a => a -> a -> (Maybe b -> ContT t m ()) -> ContT t m ()
require a b esc = if a == b then pure () else esc Nothing

blah = untilJustIterate [1, 2, 3] $ \n ->
        untilJustIterate ['a', 'b', 'c'] $ \l ->
          callCC $ \k -> Just <$> do
            require n 2 k
            require l 'a' k
            return $ show (n * 2) <> " " <> show l


untilJustIterate :: Monad m => [a] -> (a -> m (Maybe b)) -> m (Maybe b)
untilJustIterate [] f = pure Nothing
untilJustIterate (x: xs) f = f x >>= \case
                                       Nothing -> untilJustIterate xs f
                                       a -> pure a