[Someone]

> I'm interested in examining the idea of a programming language that eschews the notion of a callstack

Technically, I don’t know any language whose spec mentions the notion of a call stack. For example, it’s perfectly OK for a conforming C compiler to use a linked list of dynamically allocated activation records (from a standards view point; users of such an implementation may have different opinions)

A conforming C compiler also could choose to only use a call stack for functions that take part in recursive calls or that may get called by multiple threads.

> plus weirdo stuff like the ability to hand out references to local data in functions that have already returned (which remains valid as long as you don't call the function again, which I think should be possible to enforce via a borrow checker).

If you add multi-threading (something that is almost a must have for languages on powerful CPUs nowadays), I don’t think that’s easy to do.

[layer8]

I don’t think that “call stack” implies “contiguous memory”, or “what the operating system might think of a process call stack”, so a linked list would still qualify as a call stack. While the C standard doesn’t use the word “stack”, it explicitly requires support for recursive function calls, and the related semantics it specifies with regard to automatic storage duration effectively describe a stack.

[Someone]

If you want to ignore such implementation details, you’re basically saying you want to see a language that doesn’t allow recursion, or maybe even one that allows recursion, but only in such a way that the compiler can compute the maximum amount of memory needed for activation records.

In a language that doesn’t allow recursion, using a call stack still can make sense for the implementation because it allows reuse of memory for local variables between functions that do not call each other, directly or indirectly.

But then, you’re giving up “plus weirdo stuff like the ability to hand out references to local data in functions that have already returned”, although, thinking of it, static analysis could make those locals survive by manipulating the stack pointer (if you have the caller allocate and, reallocate the locals of the called function, the caller can postpone that reallocation if it wants to access the locals of the called function). I’m not sure that weirdo stuff is worth the effort, though. It’s just a weird way to return multiple values from a function.