List Commands Library for 50g

[John Keith]

(05-22-2018 09:40 PM)DavidM Wrote:  Are there some particular types of stack-based operations that would be helpful for this?

None that come to mind, the solution seems to be different for every problem, but I'll think about it.

Quote:Yes, if the data is to be stored in a list, there's no way to avoid a basic truth here: RPL objects are essentially COW (Copy On Write) data structures. There's no way to simply add one element to a list -- the entire list is copied to a new memory location with the change, and a pointer to that new list is pushed onto the stack where appropriate. Furthermore, any attempt to access a specific element in a list requires the command in question to play hopscotch with every preceeding element leading up to it. Those "skips" are relatively fast on the ARM-based RPL machines, but they still take time.

The RPL stack is simply an array of pointers, with each pointer being the address of some RPL object floating around in memory somewhere (usually in TEMPOB, but not always). Pointers are all the same size, so it's easy to access indexed stack levels by knowing where SL1 is and figuring out the offset to the stack level you need (and multiplying a 5-nibble number by 5 is fast and easy in Saturn code). So working with arbitrary stack levels is much faster than having to skip through each list element every time you need to access a specific item. If random access is needed (in the traditional programming sense, not "unpredictable" ), you'll almost always be better off exploding a list onto the stack. Unless the list is very large, in which case the possible GC issues that result from doing that may backfire on you.

Oh, the joys of working with lists!

That's pretty much what I feared. As I understand, this is true to some degree for all LISP-derived languages.