Re: [Haskell-cafe] Missing a "Deriving"?

30 May 2009

      Hi Miguel,

That works. but it gives just a single solution [1,2,3] when there are supposed to be two [[1,2,3],[1,4,3]]. Of course the code in YAHT may be in error.

Also, how the heck does Haskell decide which "success", "failure", "augment", and "combine" to use in function "searchAll", since there are five possibilities.

MIchael

--- On Sat, 5/30/09, Miguel Mitrofanov  wrote:

From: Miguel Mitrofanov 
Subject: Re: [Haskell-cafe] Missing a "Deriving"?
To: "michael rice" 
Cc: haskell-cafe@haskell.org
Date: Saturday, May 30, 2009, 5:36 PM

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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