On Tue, Oct 26, 2010 at 9:09 PM, Henning Thielemann
On Tue, 26 Oct 2010, Bas van Dijk wrote:
malloc :: forall a. Storable a => IO (Ptr a) malloc = mallocBytes (untag (sizeOf :: SizeOf a))
(Note that this does require the ScopedTypeVariables language extension.)
Haskell 98 solution would be nicer. Something like
malloc :: Storable a => IO (Ptr a) malloc =
let aux :: Storable a => SizeOf a -> IO (Ptr a) aux = mallocBytes . untag in aux sizeOf
Great! It's nice to know that this proposal doesn't require ScopedTypeVariables.
There's a possible 4th proposal that I'm thinking about: The Bits class from Data.Bits has the bitSize :: a -> Int method. Maybe it would be a good idea to replace that as well with: bitSize :: BitSize a; type BitSize a = Tagged a Int However I think bitSize is more often applied to an actual value than to 'undefined' so I need to investigate that a little further.
RealFloat class also provides a lot of examples that could be adapted to the Tagged scheme.
I see if I can include them in this proposal as well.
Since all of the affected modules are quite basic, this would break a lot of code. Thus I would propose a less invasive path:
1. Add functions that provide the Tagged interface and implement them using 'sizeOf' and 'alignment'.
2. Add functions that convert a sizeOfTagged and alignmentTagged function to 'sizeOf' and 'alignment' in order to allow Storable instance declaration, that not need to cope with undefined.
I agree that we should provide a comfortable upgrade path. With respect to 1 and 2, what about putting them inside the Storable class and defining them in terms of eachother: -- | Minimal complete definition: either sizeOf or sizeOfTagged, ... class Storable a where {-# DEPRECATE sizeOf "use sizeOfTagged instead" #-} sizeOf :: a -> Int sizeOf _ = untag (sizeOfTagged :: SizeOf a) sizeOfTagged :: SizeOf a sizeOfTagged = Tagged $ sizeOf (undefined :: a) {-# DEPRECATE alignment "use alignmentTagged instead" #-} alignment :: a -> Int alignment _ = untag (alignmentTagged :: SizeOf a) alignmentTagged :: SizeOf a alignmentTagged = Tagged $ alignment (undefined :: a) I don't like the name 'sizeOfTagged' though. The name I like best is just 'size :: SizeOf a'. However this is not going to work for 'alignmentTagged' because we already have 'alignment'. Any other ideas?
3. Document how to use the Tagged types with Haskell 98.
Will do. Thanks for the example.
4. In the future you may deprecate the immediate use of 'sizeOf' and 'alignment'.
We could deprecate sizeOf immediately in favor of sizeOfTagged. Deprecating it doesn't break any code but does gives warnings so that users are encouraged to upgrade. I don't see a reason to wait. Thanks, Bas