Other options may exist using packed string objects, pinning them in the GHC heap, etc. but they are not portable and probably not much more efficient.

If you use mallocForeignPtr and don't need finalizers, then I agree - the ForeignPtr method is about as efficient as PackedStrings. But if you need finalizers and real ForeignPtrs then it will be significantly less efficient in GHC. I guess it depends on whether the reference count is important for George's application. Cheers, Simon