Storable instance for Complex

[Jed Brown]

On 15 Feb 2008, wnoise at ofb.net wrote:

> On 2008-02-15, Jed Brown <jed at 59A2.org> wrote:
> > I propose that the following instance be added to base:
> >
> >   instance (RealFloat a, Storable a) =3D> Storable (Complex a) where
> >       sizeOf z    =3D 2 * sizeOf (realPart z)
> >       alignment z =3D sizeOf (realPart z)
> >       peek p =3D do
> >           [r,i] <- peekArray 2 (castPtr p)
> >           return (r :+ i)
> >       poke p (r :+ i) =3D pokeArray (castPtr p) [r,i]
> 
> I've been using a slightly different instance:
> 
> instance (RealFloat a, Storable a) => Storable (Complex a) where
>     sizeOf x    = 2 * sizeOf (f x)
>     alignment x = alignment  (f x)
>     poke      x (a :+ b) = do let y = castPtr x
>                               poke y a
>                               pokeElemOff y 1 b
>     peek      x          = do let y = castPtr x
>                               a <- peek y
>                               b <- peekElemOff y 1
>                               return (a :+ b)
> 
> f :: Complex a -> a
> f _ = undefined
> 
> With a small constant number of elements, I think it makes more sense to
> just store them explicitly rather than through a list.

I don't really care how it is written.  I suspect these generate exactly
the same code.  The important thing is that an instance with this binary
representation is put in one place since the current state prevents
certain packages from being used together.

Jed
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