Today I saw a reference to The new features of Fortran 2018 float past on Twitter. The WG5 standards committee has renamed the forthcoming language revision to Fortran 2018 from Fortran 2015 to reflect a more accurate release date. Many of the changes focus on coarrays and distributed computing but there were several useful clarifications of existing features. I was very happy to see that COMMON, EQUIVALENCE, DO <label>, FORALL, and arithmetic-IF have finally been marked as obsolescent and are on their way to the dustbin of computing practice. All but FORALL are leftovers from FORTRAN 77 and earlier. Modern Fortran provides much better and clearer facilities for persistence, aliasing, and dynamic memory management than COMMON and EQUIVALENCEand modern control structures make DO <label> and arithmetic-IF redundant. Numeric line IDs seem to attract spaghetti logic so I see the slow deprecation of these features as a welcome change to the language to improve code quality, reliability, and maintainability. When refactoring code, I tend to refactor away all four of these constructs anyway but it is heartening to see the standards committee recognize these features are problematic enough to (eventually) remove them from the language
In the case of FORALL, the construct never worked up to its potential and has since been replaced with DO CONCURRENTwhich fits more into the DO <iteration style> syntax model. FORALL is rarely used because it never reliably performed any better than a conventional loop. My early experience with modern Fortran led me to experiment with FORALL; finding that it didn’t offer any advantages on any of the compilers I used at the time led me to abandon it, though I really need to look deeper into the coarray model for concurrent execution.
If you have had experience applying coarray features to legacy code, please let me know in the comments or via email (address listed on the contact page). I’m curious what strategies you’ve found to work.