[Haskell-cafe] Missing a "Deriving"?
miguelimo38 at yandex.ru
Sat May 30 17:36:44 EDT 2009
It's trying to 'Show' the 'c [Int]' type, but doesn't know which 'c'
to use; so it's trying to find a generic instance, which doesn't
exist. You can't fix this with 'deriving' or anything like this;
instead, provide the type annotation like this:
*Main> searchAll g 1 3 :: Maybe [Int]
On 31 May 2009, at 00:50, michael rice wrote:
> 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
> It seems to be trying to 'Show' the Computation class but I'm not
> sure where to put the 'Deriving'.
> 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'),
> *Main> searchAll g 1 3
> 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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