on somewhat of a tangent, i was running into problems with where both nhc and ghc place their hi files when attempting to compile from a read-only filesystem, the standard autoconf setup (as used in DrIFT http://repetae.net/~john/computer/haskell/DrIFT) works almost perfectly except the .hi files always seem to be placed in the same directory as teh source files rather than with the .o files as would seem to make sense.
Thanks. This is a very clear statement of a similar problem I was having when building a project with GHC. I don't recall all the details now but I remember having to fight rather hard to get GHC to place files in the right place and even harder to get GHC to generate Make-style dependency files containing the correct directories. In the end I think I settled on compromising the directory structure I wanted and writing a perl script which munged the generated dependency files. The fragility of this approach was demonstrated on the next release when a slight change in the format of one of the GHC-generated files threw my Makefile and script out of whack.
Sorry I can't fill in details - this was about 18 months ago and the only records I have are some ugly-looking Makefiles. The point of my anecdote is just to add weight to the argument that this is an important issue to get right.
If there's a problem here I'd like to get it nailed. Take a look at the GHC documentation, in particular the section on flags for redirecting output: http://www.haskell.org/ghc/docs/latest/set/flag-reference.html#AEN4986 and let me know if there's anything that's not covered, or you think should be done differently. John's problem above sounds like it would be solved by using the -hidir flag. Cheers, Simon