I'm delighted!! I've been meaning to beg even more abjectly, as I've realized that the burden is significant for lots of methods or tricky ones. In some cases, where possible, I'd omit the newtype and leave my users to scary representations in their error messages.
Thank you Simon! - Conal On 9/5/07, Simon Peyton-Jones <[EMAIL PROTECTED]> wrote: > > I have heard your cry. GHC 6.8 will accept this after all. I have also > documented the rules > > > > Simon > > > > *From:* [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] *On > Behalf Of *Conal Elliott > *Sent:* 21 August 2007 21:43 > *To:* Simon Peyton-Jones > *Cc:* [email protected] > *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* <[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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