Add withForeignPtrST, touchForeignPtrST

[raehik]

withForeignPtr :: ForeignPtr a -> (Ptr a -> IO b) -> IO b is a key utility for consuming ForeignPtrs safely. It is expectably common when consuming ByteStrings.
It and its related utilities use the primops

keepAlive# :: a -> State# s -> (State# s -> b) -> b
touch# :: a -> State# s -> State# s

But withForeignPtr is stuck in IO, preventing its use in ST contexts.
(This is probably due to the relevant primops being stuck in the RealWorld until #152 , implemented in GHC 9.8.)

I propose the following new functions:

withForeignPtrST :: ForeignPtr a -> (Ptr a -> ST s b) -> ST s b
touchForeignPtrST :: ForeignPtr a -> ST s ()

These would be defined in ghc-internal at GHC.Internal.ForeignPtr, together with their IO versions.

My draft implementation below is boring, copied from the existing IO versions with ST swapped in.

withForeignPtrST :: ForeignPtr a -> (Ptr a -> ST s b) -> ST s b
withForeignPtrST fo@(ForeignPtr _ r) f = ST $ \s ->
  case f (unsafeForeignPtrToPtr fo) of
    ST action# -> keepAlive# r s action#

touchForeignPtrST :: ForeignPtr a -> ST s ()
touchForeignPtrST (ForeignPtr _ r) = touchST r

-- not exported
touchST :: ForeignPtrContents -> ST s ()
touchST r = ST $ \s -> case touch# r s of s' -> (# s', () #)

No breaking changes.

[raehik]

I originally had to write these myself in order to consume ByteStrings inside a text-builder-linear primitive, which requests ST. @bgamari was kind enough to provide a cursory check of withForeignPtrST. See Bodigrim/linear-builder#23 (comment) .