How does the FFI work?

I'm having problems with the FFI and GHC 6.6.  Namely, I edited one  
of the tools files (Detect.hs) that contains FFI imports, and now I  
end up with generalised death when I try to compile.  The source  
compiles fine, but on linking I get:

/usr/bin/ld: Undefined symbols:
_anySuspect
_extendParentSet
_findMainUse
_newParentSet
_nextChild

The C and H files are hanging around as they should be.

Any ideas what's going on?

Bob