[Haskell-cafe] Missing a "Deriving"?

[Ross Mellgren]

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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