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