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{-# LANGUAGE CPP #-}
import Control.Applicative
import Control.Monad.Logic
#if !MIN_VERSION_base(4,8,0)
import Data.Monoid (Monoid (..))
#endif
#if MIN_VERSION_base(4,9,0)
import Data.Semigroup (Semigroup (..))
#endif
parents :: [ (String, String) ]
parents = [ ("Sarah", "John")
, ("Arnold", "John")
, ("John", "Anne")
]
grandparent :: String -> Logic String
grandparent grandchild = do (p, c) <- choose parents
(c', g) <- choose parents
guard (c == c')
guard (g == grandchild)
pure p
choose = foldr ((<|>) . pure) empty
main = do let grandparents = observeAll (grandparent "Anne")
putStrLn $ "Anne's grandparents are: " ++ show grandparents
|
