[Haskell-cafe] Missing a "Deriving"?

[Ross Mellgren]

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
>
>
>
> -----Inline Attachment Follows-----
>
> _______________________________________________
> Haskell-Cafe mailing list
> Haskell-Cafe at haskell.org
> http://www.haskell.org/mailman/listinfo/haskell-cafe
>
> _______________________________________________
> Haskell-Cafe mailing list
> Haskell-Cafe at haskell.org
> http://www.haskell.org/mailman/listinfo/haskell-cafe

-------------- next part --------------
An HTML attachment was scrubbed...
URL: http://www.haskell.org/pipermail/haskell-cafe/attachments/20090601/5ef740fd/attachment.html