Simon PJ and Bulat,
[the] ForeignPtr solution [has] gotten a lot cheaper in GHC 6.6 than it used to be, so it's worth trying. A merit of the approach is that is avoids fiddling with the bignum allocator at all.
I actually did not know that until today; I have tried to keep up with the rapid changes going on but until Simon Marlow posted the FFI syntax patch on cvs-ghc-request I had not read into it that much. It won't be too much trouble for me to do a bare FFI binding to GMP or another library (people seem to be having problems with OpenSSL's license) but what I have been doing still applies: writing bitwise operators and cleaning things up. I don't know how much the indirection of a FFI binding would degrade the speed compared to a direct C-- binding (you have an extra function call with FFI); it should not be any more costly than indirection through two pointers. This will be quick: I will set up a GMP-FFI binding as a speed- reference, for starters. Best regards, Peter