On 24 May 2010 11:13, Sam Martin
Hi!
I'm trying to work out the best way to generate (ideally derive) instances for the Fix type. Here's a cut down example:
data Greet x = AlloAllo x x | AuRevoir deriving Show newtype Fix f = In { out :: f (Fix f) } -- deriving Show -- DOESN'T COMPILE
I think this is because for Fix to have a Show instance, it needs f to have a Show instance, which only works if the parameter passed to it has a Show instance, which means Fix needs to have a Show instance, etc. My guess is that the deriving tool doesn't like infinite loops...
-- workaround instance Show (Fix Greet) where show (In i) = "In " ++ show i
Are you using OverlappingInstances or something to get this to work?
In other words, given a number of parametised types that I can derive, say, Ord, Eq and Show for, how should I go about getting the instances for the Fix-d version of them as well? I've tried a few things, but no luck so far.
Does "(==) = (==) `on` out" works for the Eq instance? -- Ivan Lazar Miljenovic Ivan.Miljenovic@gmail.com IvanMiljenovic.wordpress.com