User defined functions in constant expressions

FortranFan · July 4, 2021, 4:20pm

Fyi, I’ve added an issue at the GitHub site, any comments and feedback by readers will be much appreciated:

Add CONSTEXPR procedures in Fortran

opened 04:18PM - 04 Jul 21 UTC

"**Constexpr as much as possible**" or in other words, utilize as much compile-t…ime computing as viable is a mantra that's increasingly becoming important in many modern codes based on C++17 (and later revisions) toward scientific and technical computing in industry, especially in process simulations and modeling. Improved `constexpr` in C++17 is particularly valuable when one needs to construct large named constants arrays for use in simulations based on other constants (e.g., coefficients in engineering correlations, results from prior simulation runs, results from first-principles models to be applied in reduced-order models, etc.). These are all use cases that are rather similar to some threads that have come up at Fortran-lang.org: * [User defined functions in constant expressions](https://fortran-lang.discourse.group/t/user-defined-functions-in-constant-expressions/1509?u=fortranfan) * [Thread-safe initialization of shared module data (OpenMP)](https://fortran-lang.discourse.group/t/thread-safe-initialization-of-shared-module-data-openmp/485?u=fortranfan) An option can be to introduce a **CONSTEXPR** attribute to functions in Fortran combined with appropriate semantics and constraints which can allow such a nonintrinsic i.e., user-defined functions to be used in constant expressions. Such an option appears consistent, in my view , with the evolution path of Fortran with `PURE/ELEMENTAL` procedures in Fortran 95 to [**SIMPLE**](https://j3-fortran.org/doc/year/19/19-201r1.txt) procedures in Fortran 202X. The introduction of such a **CONSTEXPR** procedure in a future Fortran standard, say 202Y, will greatly enhance compile-time computing facilities in Fortran and it holds the promise of great benefit in scientific and technical computing. Some further reading [here](https://fortran-lang.discourse.group/t/user-defined-functions-in-constant-expressions/1509/2?u=fortranfan)

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User defined functions in constant expressions

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