A traits system for Fortran is described. Its aim is to endow the language with state-of-the-art capabilities for both compile-time and run-time polymorphism, that are similar to those of the Swift, Rust, Go, or Carbon languages.

We have surveyed all these latter modern languages in order to distill their very best features with respect to polymorphism, and to tie them into a coherent and consistent package of extensions for Fortran, that is both powerful, yet easy to use (also for non-experts), and backwards compatible with the present language.

The resulting design features:

• Traits based, flexible, modern-day, OOP (as in Swift, Rust, Go)
• Traits based, fully type-checked, generics, interoperable with both procedural, functional, and OO programming (as in Swift, Rust, Go)
• Non-necessity for explicit instantiation of generics (as in Swift)
• Type sets as traits, to easily formulate generics constraints (as in Go)
• "Zero cost" static polymorphic method dispatch via generics (as in Rust)
• Interoperability with class inheritance, but also support of "sealed" classes
• Room for future growth, e.g. for future support of array-rank-genericity, or compile-time polymorphic union types