How is it so? Recursion is the only possible form of iterative control flow in the functional languages. If you do not want to allow any forms of recursion, you cannot use functional languages at all.

Of course, you can explicitly demand totality, and it's perfectly fine, but still it is quite a severe limitation, and you'll have to allow optional unrestricted recursion if you want to have (at leat local) Turing completeness.

> Recursion is mostly an academic tool.

Wrong.

> In the real world it is a bad idea.

Wrong. Your view of the real world is severely distorted.

> yet not one person offered a non-trivial use of recursion in commercial, medical or military software.

I suspect your 35 years of experience was 1 year repeated 35 times. Otherwise you would have known thousands of cases where recursion is unavoidable.

Take any real world compiler, for example. People nowadays are spoiled, they want meaningful error messages, they want semantic highlighting in IDEs, and so on. So forget about the dragon book, automatons and all such crap - your only option is a recursive descent parser.

If you want a guaranteed quality and proven correctness of your software (since you've mentioned medical and military), you have no other option but to use total languages. It's just too hard to reason about anything else. Which means - you only have recursion and no other means of control flow. Want your software to be 100% correct, robust and predictable - use recursion. I can go on forever, but things I've listed are already more than enough to debunk your point.

> It wastes resources

Wrong.

> and it simply isn't safe.

Wrong.

> Used in the wrong place it can actually kill people.

Incompetent coders kill people. Epileptoids suffering from severe forms of Dunning-Kruger effect kill people. Those who repeat 1 year of experience instead of building experience progressively kill people.

I'm seeing stack overflows (not a problem with tail-call optimization) as the biggest problem here. There are equally dangerous problems, though. Buffer overflows are a very real concern, especially in embedded software; but that doesn't mean that people don't ever use pointers. Everything about software has risks. Yes, they probably shouldn't have used recursion there in that car; but honestly, it's just a stack. Enough calls and some state could have been frozen. They should have put mitigations in place to stop the potential for a stack overflow to have actual, detrimental results. Do you refuse to ever make a function call for fear that you might be at the top of the stack somehow? After all, how can you know that you haven't accidentally created a state machine with function calls and are at the peak of it now, and end up causing the same problem. Recursion may have been the final manifestation, but it's certainly not the only place that failed.

I argue against your thought that recursion is useless, as well. It's a natural and intuitive solution for a lot of problems. The very concept of a dynamic programming solution starts with a recurrence relationship. It starts with recursion. Sorting algorithms are intutively recursive in many cases, Quicksort and Mergesort are just two examples.

And, 'dangerous' is an interesting term. I understand you've worked in embedded systems and hardware, and so those things are often critical systems that have to be super secure and completely bug free. Not every system is like that, though.

I want to understand why you believe that recursion is both useless and dangerous. Could you give some broad examples of the dangers of recursion from your years of experience?

And how exactly it is related to recursion? You should have said "memory leaks are dangerous", and everyone would agree.