Quoth wren romano
Incidentally, if we really want to pursue the "get rid of CPP by building it into the GHC distro"...
In recent years there've been a number of papers on "variational lambda-calculi"[1] which essentially serve to embed flag-based preprocessor conditionals directly into the language itself. One major benefit of this approach is that the compiler can then typecheck *all* variations of the code, rather than only checking whichever particular variation we happen to be compiling at the time. This is extremely useful for avoiding bitrot in the preprocessor conditionals.
But fatal if compilation is conditional on something that affects the ability to type check, am I right? Such as different compilers or versions of same compiler. Donn