[smcdow]

Speaking from a government contracting point of view: Nobody is going to pay you to rewrite existing code that's already working. Nobody. The customer doesn't give a flying shit about the implementation. He'd be happy with a box of diodes as an implementation, as long as it worked and came in on time and on budget.

When you're writing up your proposal for a contract or a grant, the theme should always be that you're "adding capabilities" (which should be well-defined and constrained) to the existing codebase. If you get the money, then you've got carte blanche to rewrite to your heart's content - just don't tell the customer that this is what you're doing. Just make sure that those new capabilities indeed make it into the re-write and that you introduce no regressions in the new code.

[fnord123]

People don't tend to write tests for their Fortran code so the assumption that its already working and the numbers coming out are correct is a matter of faith.

But yes, no one sees it this way.

[gmueckl]

Writing tests for the the kind of numerical code that FORTRAN is usually used for is hard. Sometimes there is no direct way of testing it because if you knew any of the results already you wouldn't need to run the simulation in the first place. Quite often, the best that you can do is proper sanity checks like conservation of energy and momentum or things like that.

[fnord123]

Yes, it's hard. But without an automated test suite checking the numbers coming out then any change to the code could introduce numerical instability.

[ehsankia]

As with any sort of big transition (changing e-mail, using a new password manager, changing programming language), the solution is always to do it incrementally.

For e-mail, I generally create a new one, and over a year or two, I create new accounts with the new e-mail and gradually move accounts until the old one is seldom used. Similarly here, it may be a bit tricky, and it really depends on how intertwined it all is, but gradually writing new pieces that you're adding in a new language, or using C++ for pieces that you're rewriting. eventually you'll be much closer than trying to do it all at once.

[andai]

This gradual approach is what Mozilla is doing with transitioning Firefox to Rust. Parts of Firefox are already written in Rust.

[noobermin]

The one thing that helps I think is writing of codes that are open source. Yes, it's a sticky point regarding getting funding, but in that imaginary world where you are funded well, transitioning to open codes (save the things that are...export controlled) would be beneficial for all of us.

I hate how I can't publish easily on modifications I make to our PIC code because it isn't open source; eventually I'm planning to switch to another code (and might implement a needed solver for it) just for the sake of my publications.