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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