Implementing fypp like pre-processing functionality in fpm

[FortranFan]

@certik

… A separate question is what it would take to rewrite fypp or something similar in functionality into Fortran, so that we can link it with fpm , so that fpm is standalone (does not depend on Python). The main target is stdlib . Let’s look at the usage in stdlib . Below I collected the main uses cases. Did I miss any?

We can use these to design a pre-processor that is possible to implement in Fortran. If these are all or most of the use cases, they can be separated into two independent kinds:

• generic over any type (either one or a combination of integer, real, complex, logical) and kind
• generic over any array rank

The first one is I think quite simple to implement (i.e., design a syntax, preferably as close to fypp as possible) and implement in Fortran. The second one is more complicated, as one has to do things like select subarrays and handle declarations properly, but still it seems very much doable. …

@certik’s idea of a preprocessor in Fortran itself is extremely relevant and important given other discussions c.f. Proposal: moving fftpack under fortran-lang - #29 by aradi

Fortran has a few crucial gaps when it comes to Generics but yet is high-level enough that the need for preprocessing to achieve generic algorithms (and containers) can be limited to particular aspects only. And it is conceivable to do with a Fortran processor-based facility that is both aware of the Fortran language semantics and syntax and can employ it effectively.