SEsedachv·Sep 8, 2017
> Which, of those statements, do you consider to be wrong, and why?
Every one of them. You do not understand the difference between what the program code does and what the programming language implementation does, and your underlying assumption is that there is a difference in how a Lisp runtime without garbage collection would manage memory allocation and how the C standard library manages memory allocation.
I am going to assume you need a stdlib.h refresher:
void *malloc(size_t size);
void free(void *ptr);
How do you think free works? How would you write a linked list implementation in C?
I just happened to have done some software archaeology on a Lisp implementation without a garbage collector a while ago (https://github.com/vsedach/Thinlisp-1.1), and the memory allocation and lifetime management is exactly the same there as
Read the rest
in C++ - manual management and dynamic extent regions (RAII).
Now I really want to get pedantic with each of your points, because individually each is wrong in interesting ways:
> 1, that every newly-created cons pair requires an allocation
This statement is a redundant tautology ("every new allocation requires an allocation"), but even the correctly phrased assertion that "CONS always allocates" is not always true:
https://en.wikipedia.org/wiki/Hash_consing
http://3e8.org/pub/scheme/doc/lisp-pointers/v1i3/p17-white.p...
> 2, that every new list entry is a new cons pair
Again, redundant, and again, not always true, because of hash consing and CDR coding:
https://en.wikipedia.org/wiki/CDR_coding
> 3, that you can avoid creating them but you have to keep a very careful eye on your code in order to do so - the normal way of writing code will cons frequently.
If you do not need to allocate lists, do not allocate lists. There is no "normal" way of writing code that exists outside of what your performance requirements are, what the language semantics are, and what the language implementation provides in terms of those semantics. The only situation where you cannot avoid frequent allocations is when all the data structures are immutable.