Another way to do this would be to have GHC --make invoke itself to compile each module separately. Actually I think I prefer this method, although it might be a bit slower since each individual compilation has to read lots of interface files. The main GHC --make process would do the final link only. A fun hack for somebody?
this would also help building large libraries on architectures with little memory, as it seems to me that when one ghc instance is compiling multiple modules in a row, some leaked memory/unevaluated thunks pile up and eventually cause the compilation to abort. I suspect that building each file on its own avoids this issue.
In my experience, reading all those .hi files is not so quick, about 1.5s for around 200 modules, on an SSD. It gets worse with a pgmF, since ghc wants to preprocess each file, it's a minimum of 5s given 'cat' as a preprocessor. Part of my wanting to use make instead of --make was to avoid this re-preprocessing delay. It's nice that it will automatically notice which modules to recompile if a CPP define changes, but not so nice that it has to take a lot of time to figure that out every single compile, or for a preprocessor that doesn't have the power to change whether the module should be recompiled or not.