On Monday, 11 July 2016 at 14:36:22 UTC, Andrew Godfrey wrote:
On Monday, 11 July 2016 at 10:25:36 UTC, Tofu Ninja wrote:
On Sunday, 10 July 2016 at 13:15:38 UTC, Andrew Godfrey wrote:
Btw here's a thread from 2014 that touches on the idea of a "tailrec" annotation. At the time, Walter viewed the optimization as the compiler's business and not something he'd elevate to a language feature:



http://forum.dlang.org/post/lqp6pu$1kkv$1...@digitalmars.com

I find it odd that something like tail recursions that actually makes certain things possible that wouldn't be otherwise is seen as only an optimization, when things like inlining which really is only an optimization is seen as important enough to have a pragma.

I agree. Maybe Walter has reconsidered since then. He did also say, though, that he thinks D supports enough different programming styles already.

Would you be satisfied with a pragma? I'd intuited (but could be wrong) that the focus of your proposal was to get a compiler error if someone makes a change to a recursive function, that causes the compiler to be unable to apply the TCO optimization. If that is your focus, it has heavy implications and the feature can't just be a pragma.

Pragma does not seem enough, at least current pragmas can be ignored by the compilers (http://dlang.org/spec/pragma.html#predefined-pragmas), but if it's required for tco it can make it.


I've been thinking about changing @tco for @boundedStack, as it'll really reflect guarantees on functions while implicitly asking for TCO on functions that require it. But the fact that most functions should be marked as @boundedStack is something that bothers me. The complement, @unboundedStack, might be better, as it would explicitly mark the functions that might cause stack overflows so they drag required attention and would also force the compiler to do TCO where required. Focusing on stack size, rather than directly on TCO might even allow to guarantee that many D programs do not crash due to a stack overflow, which is a nice guarantee that not many languages, allow to express.
I'd like thoughts on this change of perspective.


BTW, I'm glad that discussion has arised, as it can only make the propossal better.
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