Jason Dagit <
dagit@codersbase.com> writes:
> On Fri, Apr 30, 2010 at 11:08 PM, Ivan Lazar Miljenovic <
>
ivan.miljenovic@gmail.com> wrote:
>>
>>
>> You're putting the constraint in the wrong places: put the "(Cls a) => "
>> in the actual functions where you need it.
>>
>
> That's solid advice in general, but it's still not going to work here if any
> of the functions needed for the instance of Graph require the type class
> constraint.
about the constraint.
Perhaps this "working" example illustrates the change I want to make. Working in the sense that it type checks but it's a silly example just to illustrate the point:
{-# LANGUAGE MultiParamTypeClasses #-}
{-# LANGUAGE FlexibleInstances #-}
module Graph where
import Data.Graph.Inductive.Graph hiding (Graph)
-- Defive some arbitrary class, and give it a 'boring'
-- reason to use it.
class Cls a where
boring :: a
data Blah = Blah
-- Make sure we have at least one instance, but not really needed for this example
instance Cls Blah where
boring = Blah
data B a = B [a]
data GrB a b = GrB (B a)
-- Just copy the bits from FGL that are interesting here
class Graph gr a b where
empty :: gr a b
-- | True if the given 'Graph' is empty.
isEmpty :: gr a b -> Bool
-- | Create a 'Graph' from the list of 'LNode's and 'LEdge's.
mkGraph :: [LNode a] -> [LEdge b] -> gr a b
-- | A list of all 'LNode's in the 'Graph'.
labNodes :: gr a b -> [LNode a]
instance Cls a => Graph GrB a b where
empty = GrB (B [boring])
isEmpty (GrB (B [])) = True
isEmpty _ = False
mkGraph _ _ = GrB (B [])
labNodes _ = []
The Graph class is actually unchanged other than mentioning 'a' and 'b'. This mention of 'a' and 'b' allows instance writers to add contexts other than () when defining instances.