Oh I wasn't clear -- you need multiple instance declarations for a given type (Failable, for example), one for each type class you're implementing.

That is,

instance Monad Failable where

return = ...

...

instance MonadPlus Failable where

mplus = ...

...

-Ross

On Jun 1, 2009, at 9:40 PM, michael rice wrote:

Hi Ross,

I thought of that, but return, fail, and >>= became "not visible" when I changed the instance declaration from Monad to MonadPlus.. Can Failable be in two instance declarations, one for Monad (giving it return, fail, and >>=) and one for MonadPlus (giving it mplus)?

Michael

--- On Mon, 6/1/09, Ross Mellgren <rmm-haskell@z.odi.ac> wrote:

From: Ross Mellgren <rmm-haskell@z.odi.ac>
Subject: Re: [Haskell-cafe] Missing a "Deriving"?
To: "michael rice" <nowgate@yahoo.com>
Cc: "haskell-cafe Cafe" <haskell-cafe@haskell.org>
Date: Monday, June 1, 2009, 9:33 PM

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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