Re: [Haskell-cafe] NFData question

On Fri, Sep 4, 2009 at 12:57 PM, Peter Verswyvelen wrote:

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

I think you need your definition here because if you had: Vec2 (String, String) I think the declaration of Vec2 just ensures that the tuple is constructed. But to ensure that the strings are evaluated, you need to recursively call rnf (I think). That's my understanding, but I could be wrong. Jason