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 <[EMAIL PROTECTED]> 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:* [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] *On
> Behalf Of *Conal Elliott
> *Sent:* 15 August 2007 23:37
> *To:* [email protected]
> *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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