Re: Storable instance for Complex

15 Feb 2008


      On 15 Feb 2008, wnoise@ofb.net wrote:
...
On 2008-02-15, Jed Brown  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