Re: [Haskell-cafe] Missing a "Deriving"?

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 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@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 wrote:

From: Ross Mellgren Subject: Re: [Haskell-cafe] Missing a "Deriving"? To: "michael rice" Cc: "haskell-cafe Cafe" 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 wrote:

From: Ross Mellgren Subject: Re: [Haskell-cafe] Missing a "Deriving"? To: "michael rice" Cc: "haskell-cafe Cafe" 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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