On 15 Feb 2008, lemming@henning-thielemann.de wrote:
On Fri, 15 Feb 2008, Jed Brown wrote:
I propose that the following instance be added to base:
instance (RealFloat a, Storable a) => Storable (Complex a) where sizeOf z = 2 * sizeOf (realPart z) alignment z = sizeOf (realPart z) peek p = do [r,i] <- peekArray 2 (castPtr p) return (r :+ i) poke p (r :+ i) = pokeArray (castPtr p) [r,i]
This instance is binary compatible with C99, C++ and Fortran complex types.
It is currently needed by at least two independent packages: hmatrix and fft. Since it is natural for user code to use both of these packages, we need to move the instance to a common location.
I also need it for signal processing.
In light of the fact that several packages need this instance, I created a package storable-complex (on Hackage) which has the instance in Foreign.Storable.Complex. In the interest of enabling our packages to play nicely *now*, I would like to encourage everyone needing this instance to get it from this common source. When it (hopefully) makes it into base, we can use a flag in the .cabal file to get it from base instead of this package. Note on style: I decided I agree with Aaron about using explicit peek/poke rather than a list. Thus the instance in storable-complex is written: instance (RealFloat a, Storable a) => Storable (Complex a) where sizeOf z = 2 * sizeOf (realPart z) alignment z = sizeOf (realPart z) peek p = do let q = castPtr p r <- peek q i <- peekElemOff q 1 return (r :+ i) poke p (r :+ i) = do let q = (castPtr p) poke q r pokeElemOff q 1 i Is Storable/Complex really a special case, or are there other cases where equivalent instances are defined in different packages? If so, would a language extension allowing the user (importing the same instance from multiple sources) to specify that the instances are actually identical be helpful? Jed