Monad

gistfile1.hs

type Choice a = [a]

choose :: [a] -> Choice a
choose xs = xs

-- Join
join :: Choice (Choice a) -> Choice a
join choices = concat choices

-- Bind
(>>=) :: Choice a -> (a -> Choice b) -> Choice b
choices >>= f = join (map f choices)

-- Bind (ignore original)
(>>) :: Choice a -> Choice b -> Choice b
choices >> f = choices >>= \_ -> f

-- Return
return :: a -> Choice a
return x = choose [x]

-- Guard
mzero :: Choice a
mzero = choose []

guard :: Bool -> Choice ()
guard True  = return ()
guard False = mzero

test' :: Choice (Int, Int, Int)
test' = do
        x <- choose [1,2,3]
        y <- choose [4,5,6]
        z <- choose [7,8,9]
        guard ((x + y) * z == 45)
        return (x, y, z)

main = do
        print(take 1 test')