[Haskell-cafe] NFData question

[Daniel Fischer]

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