Thanks for your help, Duncan. On Jan 12, 2009, at 6:10 AM, Duncan Coutts wrote:
Do the (ForeignPtr e) and the (Ptr e) point to the same thing? They appear to be related because you dereference p but touch f.
It used to be the ForeignPtr was slower to dereference than a Ptr and so caching it like this could make it faster (but then requires lots of careful thought about touchForeignPtr). These days ForeignPtr is just as fast and so you can use withForeignPtr and never have to worry about touchForeignPtr. You'll notice ByteString works this way.
Often they point to the same thing, but not always. If they pointed to the same thing, I wouldn't store p separately, but often p has a nonzero offset from f. You either have to store the offset or cache the pointer. I found that it was both faster and simpler to cache p rather than the offset. Patrick