That's great, thanks. Looks like FlexibleContexts is redundant (effectively a subset of UndecidableInstances?). Ivan, I hadn't realised, but I had FlexibleInstances on before for other reasons. I guess that's why I ccould get the workaround to compile. Cheers, Sam -----Original Message----- From: Reid Barton [mailto:rwbarton@math.harvard.edu] Sent: Mon 24/05/2010 02:28 To: Sam Martin Cc: haskell-cafe@haskell.org Subject: Re: [Haskell-cafe] Best way to instance Fix? On Mon, May 24, 2010 at 02:13:32AM +0100, Sam Martin wrote:
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
-- workaround instance Show (Fix Greet) where show (In i) = "In " ++ show i
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.
Any clues?
You can use GHC's standalone deriving mechanism for this, described at http://www.haskell.org/ghc/docs/6.12.2/html/users_guide/deriving.html {-# LANGUAGE StandaloneDeriving, FlexibleContexts, UndecidableInstances #-} data Greet x = AlloAllo x x | AuRevoir deriving Show newtype Fix f = In { out :: f (Fix f) } deriving instance Show (f (Fix f)) => Show (Fix f) Regards, Reid Barton