I went back and tried to convert the YAHT example to Monad, importing Monad, commenting out all but the data descriptions and the searchAll function, and finally replacing success, failure, augment, and combine in the searchAll function with return, fail, >>=, and mplus. *Main> let g = Graph [(1,'a'),(2,'b'),(3,'c'),(4,'d')] [(1,2,'p'),(2,3,'q'),(1,4,'r'),(4,3,'s')] *Main> searchAll g 1 3 :: [[Int]] [[1,2,3],[1,4,3]] *Main> searchAll g 1 3 :: Maybe [Int] Just [1,2,3] *Main> searchAll g 3 1 :: Maybe [Int] Nothing *Main> searchAll g 3 1 :: [[Int]] [] All good so far, but then tried to convert Failable from Computation to Monad instance Monad Failable where     return = Success     fail = Fail     >>= (Success x) f = f x     >>= (Fail s) _ = Fail s     mplus (Fail _) y = y     mplus x _ = x   and got the following error. Prelude> :l graph5 [1 of 1] Compiling Main             ( graph5.hs, interpreted ) graph5.hs:34:4: parse error on input `>>=' Failed, modules loaded: none. Prelude> Complete code follows. Michael ========================= import Monad data Failable a = Success a | Fail String deriving (Show) data Graph v e = Graph [(Int,v)] [(Int,Int,e)] {- class Computation c where     success :: a -> c a     failure :: String -> c a     augment :: c a -> (a -> c b) -> c b     combine :: c a -> c a -> c a instance Computation Maybe where     success = Just     failure = const Nothing     augment (Just x) f = f x     augment Nothing _ = Nothing     combine Nothing y = y     combine x _ = x instance Computation Failable where     success = Success     failure = Fail     augment (Success x) f = f x     augment (Fail s) _ = Fail s     combine (Fail _) y = y     combine x _ = x -} instance Monad Failable where     return = Success     fail = Fail     >>= (Success x) f = f x     >>= (Fail s) _ = Fail s     mplus (Fail _) y = y     mplus x _ = x {- instance Computation [] where     success a = [a]     failure = const []     augment l f = concat (map f l)     combine = (++) searchAll g@(Graph vl el) src dst     | src == dst = success [src]     | otherwise = search' el     where search' [] = failure "no path"           search' ((u,v,_):es)               | src == u = (searchAll g v dst `augment`                              (success . (u:)))                             `combine` search' es               | otherwise = search' es -} searchAll g@(Graph vl el) src dst     | src == dst = return [src]     | otherwise = search' el     where search' [] = fail "no path"           search' ((u,v,_):es)               | src == u = (searchAll g v dst >>=                              (return . (u:)))                             `mplus` search' es               | otherwise = search' es