Proposal: Don't require users to use undefined

[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


> 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.


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.

3. Document how to use the Tagged types with Haskell 98.

4. In the future you may deprecate the immediate use of 'sizeOf' and 
'alignment'.