Modern Fortran submodule vs. preprocessor
Fortran has a long legacy of preprocessing systems, reaching back at least to the 1970s when C was not yet widely known.
I would argue that modern Fortran ≥ 2008 has eliminated most preprocessors use cases.
It’s usually much cleaner and quicker (build/runtime) to use Fortran
submodule to switch functionality such as
MKL vs. Atlas vs. Netlib Lapack
using CMake, for example.
Numerous Python preprocessors have arisen (and sunk) along with the usual C/C++-based preprocessors.
The only thing we use preprocessors for is very simple single-line statements, where adding a
submodule would be significantly more cumbersome than a one-liner preprocessor statement.
Historically, very elaborate Fortran preprocessors were developed such that they nearly became a language unto themselves.
FLECS and Ratfor (also ratfiv, ratfor90, etc.)
These extensive preprocessors have been made obsolete by advances from Fortran 77 onward.