On 19 February 2012 13:12, Maxime Henrion <mhenr...@gmail.com> wrote:
> Any suggestions are welcome.
Nice work but it would be nice to have this functionality directly in
the deepseq package as in:
#ifdef GENERICS
{-# LANGUAGE DefaultSignatures, TypeOperators, FlexibleContexts #-}
#endif
class NFData a where
rnf :: a -> ()
rnf a = a `seq` ()
#ifdef GENERICS
default rnf :: (Generic a, GNFData (Rep a)) => a -> ()
rnf = grnf . from
class GNFData f where
grnf :: f a -> ()
instance GNFData U1 where
grnf U1 = ()
{-# INLINE grnf #-}
instance NFData a => GNFData (K1 i a) where
grnf = rnf . unK1
{-# INLINE grnf #-}
instance GNFData f => GNFData (M1 i c f) where
grnf = grnf . unM1
{-# INLINE grnf #-}
instance (GNFData f, GNFData g) => GNFData (f :+: g) where
grnf (L1 x) = grnf x
grnf (R1 x) = grnf x
{-# INLINE grnf #-}
instance (GNFData f, GNFData g) => GNFData (f :*: g) where
grnf (x :*: y) = grnf x `seq` grnf y
{-# INLINE grnf #-}
#endif
Unfortunately this is not possible since the two default
implementations conflict. I see two solutions:
1) Change the DefaultSignatures extension to always give preference to
the default signature. I think giving preference to the default
signature makes sense since it's usually more specific (more
constraint) and thus "more correct" than the default implementation.
2) Remove the default implementation of rnf. I understand the default
implementation gives some convenience when writing instances for types
that have an all strict representation, as in:
instance NFData Int
instance NFData Word
instance NFData Integer
...
However, I think having the default implementation can mask some bugs as in:
data T = C Int; instance NFData T
which will neither give a compile time error nor warning.
I don't think it's that much more inconvenient to write:
instance NFData Int where rnf = rnf'
instance NFData Word where rnf = rnf'
instance NFData Integer where rnf = rnf'
...
where
rnf' :: a -> ()
rnf' a = a `seq` ()
So I would vote for option 2, removing the default rnf implementation.
If I find some time I will turn this into an official proposal.
Regards,
Bas
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