Steve Severance
Moving from x86 to x64 has advantages and disadvantages from my POV. Advantages: * Able to address more memory * More registers for code generation * Haskell dependencies wouldn't need to be built for x86 on Snow Leopard (though if we swapped to x64 on Leopard as well, the Leopard users would start having to build 64-bit libraries specially)
If we keep the capability to build both, and come up with a good way to use the platform default, this would be the biggest advantage for OS X users. (Full disclosure: speaking as a Snow Leopard user). Also, as time goes on, we will be seeing more, not less, 64-bit OS X users and releases.
Disadvantages: * Pointers become wider, and Haskell data structures mostly consist of pointers. This will bloat memory use of Haskell programs.
I'd be interested to see how much this happens in practice, but it does seem to be a big concern.
* Generated binaries won't work on older Macs that don't have a 64-bit OS/CPU. This is important if you are distributing compiled Haskell binaries, which is not something I personally do but which is probably important to support
The compilers/linkers for most languages supported on OS X support generating "universal" binaries with both 32- and 64- bit code. I wonder if this is something GHC could support?