RE: [Haskell-cafe] Best way to instance Fix?
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:rwbar...@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
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Re: [Haskell-cafe] Best way to instance Fix?
On 24 May 2010 11:13, Sam Martin sam.mar...@geomerics.com 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
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.miljeno...@gmail.com
IvanMiljenovic.wordpress.com
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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
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