Missing a "Deriving"?
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
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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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'.
My guess is that your expression is polymorphic, returning Computation c => c [Int] Since Haskell knows how to show this for both of the instances defined for Computation (i.e. Maybe [Int] and [[Int]]), perhaps you could give one of those as an explicit type signature? -k -- If I haven't seen further, it is by standing in the footprints of giants
participants (3)
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Ketil Malde -
michael rice -
Miguel Mitrofanov