Am Freitag 04 September 2009 21:57:27 schrieb Peter Verswyvelen:

When ones makes an ADT with data constructors that has strict (and maybe unpacked) fields,

e.g.

data Vec2 a = Vec2 {-# UNPACK #-} !a {-# UNPACK #-} !a

how does one define an NFData instance?

Like this?

instance NFData a => NFData (Vec2 a) where rnf (Vec2 x y) = rnf x `seq` rnf y

Yep.

Or is it enough to just do instance NFData a => NFData (Vec2 a)

since Vec2 is fully strict anyway, so that default rnf implementation will do?

Not necessarily. It will do if a is a simple type for which whnf == nf, like Int, but otherwise the components of Vec2 are only forced to whnf by the strictness annotations and the default implementation of rnf won't do anything more. module Vec2 where import Control.Parallel.Strategies data Vec2 a = Vec2 {-# UNPACK #-} !a {-# UNPACK #-} !a deriving Show instance NFData (Vec2 a) ghci> let v = Vec2 [True,False] [False,True,undefined] ghci> case v `using` rnf of { Vec2 l1 l2 -> (l1,take 2 l2) } ([True,False],[False,True]) ghci> v Vec2 [True,False] [False,True,*** Exception: Prelude.undefined

Thanks, Peter