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