unfoldM

[Mark T.B. Carroll]

I wanted a monadic unfoldr and couldn't see one so I thought I'd share two
possibilities I'd been playing with:


unfoldM_a :: Monad m => (b -> Maybe (m (a, b))) -> b -> m [a]

unfoldM_a f base =
    case f base of
	Nothing     -> return []
	Just result -> do (a, b) <- result
			  rest <- unfoldM_a f b
			  return (a : rest)

unfoldM_b :: Monad m => (b -> m (Maybe (a, b))) -> b -> m [a]

unfoldM_b f base =
    do fb <- f base
       case fb of
	   Nothing     -> return []
	   Just (a, b) -> do rest <- unfoldM_b f b
			     return (a : rest)

test_case_a "" = Nothing
test_case_a word@(_:xs) = Just (do a <- print (length word)
			           return (a, xs))

test_case_b "" = return Nothing
test_case_b word@(_:xs) = do a <- print (length word)
			     return (Just (a, xs))

main = unfoldM_a test_case_a "foo"
       putStr "----\n"
       unfoldM_b test_case_b "foo"


-- Mark