+1
I'm more than happy to strip the Data.Default instance from Data.Tagged,
I've already had a couple of users complain about how 'heavy' a dependency
it is, since Data.Default has to provide all the definitions for lots of
other libraries that predate it, including the MTL.
-Edward
On Tue, Oct 26, 2010 at 12:15 PM, Bas van Dijk
Dear all,
Users of the sizeOf or alignment methods (:: a -> Int) of the Storable class are pushed to use 'undefined' in their programs. Take the following function from Foreign.Marshal.Alloc as an example:
malloc :: Storable a => IO (Ptr a) malloc = doMalloc undefined where doMalloc :: Storable b => b -> IO (Ptr b) doMalloc dummy = mallocBytes (sizeOf dummy)
I find the use of 'undefined' ugly; its only purpose is to help the type-checker by carrying around a type variable. It also makes the job of an optimizing compiler harder because, in order to avoid generating unnecessary code, the compiler needs to find out if undefined isn't used. More importantly however, 'undefined' is dangerous; The type-checker will never complain when you accidentally use 'undefined' in the wrong place increasing the change that your program will crash. Also, instance writers for the Storable class need to be careful not to evaluate the argument of sizeOf because it may be undefined.
The use of 'undefined' is not only required by the Storable class. Users of the HasResolution class from Data.Fixed also need to use 'undefined' in order to get the resolution of a fixed value:
class HasResolution a where resolution :: p a -> Integer
I would like to propose solving this. My proposal consists of 3 sub-proposals:
1) Add module Data.Tagged. 2) Modify the sizeOf and alignment methods of the Storable class. 3) Modify the HasResolution class.
What follows are more detailed explanations of the proposals:
1) Add module Data.Tagged.
My proposal is to move the Data.Tagged module from Edward A. Kmett's tagged package to base. See: http://hackage.haskell.org/package/tagged The only modification that needs to be done is to drop the Default instance for Proxy because this will otherwise require that Data.Default be moved to base as well which isn't my intention. When this proposal is accepted Data.Default can provide the instance instead.
2) Modify the sizeOf and alignment methods of the Storable class.
I would like to replace the following:
class Storable a where sizeOf :: a -> Int alignment :: a -> Int
with:
class Storable a where sizeOf :: SizeOf a alignment :: Alignment a
type SizeOf a = Tagged a Int type Alignment a = Tagged a Int
To retrieve the actual size of type 'a' use: untag (sizeOf :: SizeOf a) where: untag :: Tagged s b -> b from Data.Tagged.
See the following for the haddock documentation:
http://code.haskell.org/~basvandijk/doc/ghc/html/libraries/base-4.3.0.0/Fore...
Here's the definition of the previous malloc function to give you an impression how code looks when my proposals are accepted:
malloc :: forall a. Storable a => IO (Ptr a) malloc = mallocBytes (untag (sizeOf :: SizeOf a))
(Note that this does require the ScopedTypeVariables language extension.)
3) Modify the HasResolution class.
I would like to modify the HasResolution class in the same way. So replacing:
class HasResolution a where resolution :: p a -> Integer
with:
class HasResolution a where resolution :: Resolution a
type Resolution a = Tagged a Integer
See the following for the haddock documentation:
http://code.haskell.org/~basvandijk/doc/ghc/html/libraries/base-4.3.0.0/Data...
Note that Fixed also gets a HasResolution instance:
instance HasResolution a => HasResolution (Fixed a) where resolution = retag (resolution :: Resolution a)
where: retag :: Tagged s b -> Tagged t b from Data.Tagged.
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.
A patch for the base package is attached to the ticket. I also attached patches for the ghc compiler, bytestring, binary, vector and dph. The latter are just some packages that were inside my ghc repository. This email is CCed to the maintainers of these packages.
Deadline: 3 weeks from now (Tuesday 16 November 2010) but I will shift it when the discussion demands it.
Ticket: http://hackage.haskell.org/trac/ghc/ticket/4443
Regards,
Bas