mplus is a method of class MonadPlus, so you need to write it in a separate instance from the one for Monad, e.g. instance MonadPlus Failable where mplus = ... -Ross On Jun 1, 2009, at 9:28 PM, michael rice wrote:

Still stumped. Maybe and [] are in the same MonadPlus monad, but how do I make monad Failable understand mplus?

I'm now getting this error upon loading:

Prelude> :l graph5 [1 of 1] Compiling Main ( graph5.hs, interpreted )

graph5.hs:36:4: `mplus' is not a (visible) method of class `Monad' 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

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