[Haskell-cafe] Missing a "Deriving"?

[Ryan Ingram]

> graph5.hs:37:9:
>     Warning: No explicit method nor default method for `mzero'
>     In the instance declaration for `MonadPlus Failable'

This warning is saying you didn't finish the declaration.

Try something like

instance MonadPlus Failable where
    mplus (Fail _) y = y
    mplus x _ = x

    mzero = Fail "mzero"

Also, I'd use "import Control.Monad" instead of "import Monad".

  -- ryan

On Mon, Jun 1, 2009 at 7:03 PM, michael rice <nowgate at yahoo.com> wrote:
> I didn't know I could do that. Works fine. Output below. Thanks!
>
> This is some pretty neat stuff, and I've only scratched the surface.
>
> Michael
>
> ===================
>
> [michael at localhost ~]$ ghci
> GHCi, version 6.10.1: http://www.haskell.org/ghc/  :? for help
> Loading package ghc-prim ... linking ... done.
> Loading package integer ... linking ... done.
> Loading package base ... linking ... done.
> Prelude> :l graph5
> [1 of 1] Compiling Main             ( graph5.hs, interpreted )
>
> graph5.hs:37:9:
>     Warning: No explicit method nor default method for `mzero'
>     In the instance declaration for `MonadPlus Failable'
> Ok, modules loaded: Main.
> *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 :: Failable [Int]
> Success [1,2,3]
> *Main> searchAll g 3 1 :: Failable [Int]
> Fail "no path"
> *Main> searchAll g 1 3 :: Maybe [Int]
> Just [1,2,3]
> *Main> searchAll g 3 1 :: Maybe [Int]
> Nothing
> *Main> searchAll g 1 3 :: [[Int]]
> [[1,2,3],[1,4,3]]
> *Main> searchAll g 3 1 :: [[Int]]
> []
> *Main>
>
>
>
> --- On Mon, 6/1/09, Ross Mellgren <rmm-haskell at z.odi.ac> wrote:
>
> From: Ross Mellgren <rmm-haskell at z.odi.ac>
> Subject: Re: [Haskell-cafe] Missing a "Deriving"?
> To: "michael rice" <nowgate at yahoo.com>
> Cc: "haskell-cafe Cafe" <haskell-cafe at haskell.org>
> Date: Monday, June 1, 2009, 9:43 PM
>
> 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 at z.odi.ac> wrote:
>
> From: Ross Mellgren <rmm-haskell at z.odi.ac>
> Subject: Re: [Haskell-cafe] Missing a "Deriving"?
> To: "michael rice" <nowgate at yahoo.com>
> Cc: "haskell-cafe Cafe" <haskell-cafe at 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
>
>
> -----Inline Attachment Follows-----
>
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