ByteString-backed Handles, and another couple of questions

Simon Marlow marlowsd at
Wed Dec 16 04:26:40 EST 2009

On 16/12/09 03:02, Duncan Coutts wrote:
> On Tue, 2009-12-15 at 12:48 -0800, Bryan O'Sullivan wrote:
>> Yes, that would amount to double-buffering, and would work nicely,
>> only the current buffers go through foreign pointers while text uses
>> an unpinned array. I can see why this is (so iconv can actually work),
>> but it does introduce a fly into the ointment :-)
> It should not be strictly necessary to use a ForeignPtr in this case. If
> the IO buffers use pinned ByteArray#s then they can still be passed to
> iconv for it to write into.
> It should also be possible for Text to be constructed from a pinned
> ByteArray#.

I don't think there's any real difficulty here.  The IO buffers are 
ForeignPtrs because we want the flexibility of being able to use mmap'd 
memory - the mmap-backed Handle implementation uses that.  But in normal 
operation these ForeignPtrs have pinned ByteArray#s inside.

We could provide a function of type ForeignPtr -> IO (Maybe ByteArray), 
and the text package can turn that into a chunk of Text.  It would even 
be able to support reading Text from mmaped memory, because only the 
byte buffer needs to be mmaped, not the Char buffer.


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