change to deriving in 6.7 ??

[Simon Peyton-Jones]

I have heard your cry.  GHC 6.8 will accept this after all.  I have also documented the rules

Simon

From: conal.elliott at gmail.com [mailto:conal.elliott at gmail.com] On Behalf Of Conal Elliott
Sent: 21 August 2007 21:43
To: Simon Peyton-Jones
Cc: cvs-ghc at haskell.org
Subject: Re: change to deriving in 6.7 ??

Thanks, Simon.  The manual deriving is easier than I expected, by boilerplate delegation of constraints & methods, as below.  You may want to give such an example when you document the change.   Cheers,  - Conal

-- | Pairing of type constructors
newtype (f :*: g) a = Prod { unProd :: (f a, g a) }
  -- deriving (Show, Eq, Ord)

-- Deriving no longer works on types like :*:.  So:
instance (Show (f a, g a)) => Show ((f :*: g) a) where
  show (Prod p) = "Prod " ++ show p
instance (Eq (f a, g a)) => Eq ((f :*: g) a) where
  Prod p == Prod q = p == q
instance (Ord (f a, g a)) => Ord ((f :*: g) a) where
  Prod p < Prod q = p < q

On 8/20/07, Simon Peyton-Jones <simonpj at microsoft.com<mailto:simonpj at microsoft.com>> wrote:

Yes, this is a change.  To guarantee that we generate terminating instance declarations, GHC now insists that it can reduce the context of a derived instance to

instance (C a, D b) => ...

That is, the context of the instance decl must consist of (class applied to type variables).  Without this constraint it's all too easy to generate non-terminating instances.



So the deriving mechanism works in straightforward cases, and for more complicated cases you have to write the instances yourself.



It's not impossible that the deriving mechanism could be made a little more liberal, but it errs on the conservative side at the moment.  There's no workaround - -you just have to write the instance yourself.



I should document this



Simon



From: cvs-ghc-bounces at haskell.org<mailto:cvs-ghc-bounces at haskell.org> [mailto: cvs-ghc-bounces at haskell.org<mailto:cvs-ghc-bounces at haskell.org>] On Behalf Of Conal Elliott
Sent: 15 August 2007 23:37
To: cvs-ghc at haskell.org<mailto:cvs-ghc at haskell.org>
Subject: change to deriving in 6.7 ??



I'm running ghc-6.7.20070802 and getting a new error message that didn't show up with ghc-6.6.  Code:

    -- | Pairing for unary type constructors.
    newtype Pair1 f g a = Pair1 {unPair1 :: (f a, g a)}
      deriving (Eq, Ord, Show)

Error message:

    src/Data/Tupler.hs:26:0:
    No instances for (Show (g a), Show (f a))
      arising from the 'deriving' clause of a data type declaration
               at src/Data/Tupler.hs:(26,0)-(27,25)
    Possible fix:
      add an instance declaration for (Show (g a), Show (f a))
    When deriving the instance for (Show (Pair1 f g a))

Has there been a change to "deriving"?  Is there a workaround?

Thanks,  - Conal

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