Re: Haskell Platform proposal: Add the vector package

On 19/06/2012, at 00:03, Johan Tibell wrote:

On Mon, Jun 18, 2012 at 3:53 PM, Bas van Dijk wrote:

...

Slicing is done by directly updating the pointer in the ForeignPtr:

{-# INLINE basicUnsafeSlice #-} basicUnsafeSlice i n (Vector _ fp) = Vector n (updPtr (`advancePtr` i) fp)

{-# INLINE updPtr #-} updPtr :: (Ptr a -> Ptr a) -> ForeignPtr a -> ForeignPtr a updPtr f (ForeignPtr p c) = case f (Ptr p) of { Ptr q -> ForeignPtr q c }

This saves an Int.

(This is off-topic as far as the proposal.)

ByteString has an extra Ptr field so that accessing the data is fast, given that ForeignPtrs can't be unpacked.

ForeignPtrs can be unpacked, just not manually (which is a GHC bug, IMO, I should report it): data Vector a = Vector {-# UNPACK #-} !Int {-# UNPACK #-} !(ForeignPtr a) ByteString just has some room for optimisation here ;-) Roman