On 2015-05-21 at 16:54:11 +0200, Bardur Arantsson wrote: [...]
That would be the preferred way from a technical standpoint, as it would avoid fork/exec and make it easier to integrate the CPP-phase tighter into the lexer/parser.
fork/exec is almost certainly going to be negligable compared to the overall compile time anyway. It's not like GHC is fast enough for it to matter.
Performance isn't (my) motivation for avoiding fork/exec (and the equivalent on Win32) but rather avoiding the added complexity of marshalling/IPC with fork/exec, as opposed to simply calling into a native Haskell function and crossing process boundaries and having to deal with the various things that can go wrong with the additional moving parts you encounter when controlling an external process. So this would IMO simplify code paths, and moreover I'd expect opportunities to actually make the Haskell cpphs API richer (in case it isn't already) and more tailored to GHC's lexer/parser pipeline and error-reporting.