[Haskell-cafe] Missing a "Deriving"?

[michael rice]

The following code is from Section 8.4.2, pgs. 111-112 (PDF paging) of YAHT.

It compiles fine, but upon trying it I get the following error message.

It seems to be trying to 'Show' the Computation class but I'm not sure where to put the 'Deriving'.

Michael


============

Loading package ghc-prim ... linking ... done.
Loading package integer ... linking ... done.
Loading package base ... linking ... done.
[1 of 1] Compiling Main             ( graph4.hs, interpreted )
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

<interactive>:1:0:
    No instance for (Show (c [Int]))
      arising from a use of `print' at <interactive>:1:0-14
    Possible fix: add an instance declaration for (Show (c [Int]))
    In a stmt of a 'do' expression: print it

============================

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 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




      
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