Hello,
Freeman Gilmore <
address@hidden> ezt írta (időpont: 2020. márc. 20., Pén 1:45):
Hi:
I
am not a programmer. Have some understanding
of scheme. I have a question that I cannot find the answer
to on the net. I joined this group to
ask if someone would help me find the answer.
I
am looking for a manual or whatever that will explain in detail how an alist is
stored in memory and how it works at this level. Also, how assq and
assq-ref work, what is past to the alist at the same level. Also, the source code would be good.
Storing an atomic value is implementation dependent, but usually implemented by a type tagged structure.
(You can think about this as a tuple of a type, buffer, where buffer is a length,pointer there are several optimizations, but this is a possibility).
A pair is a structure holding two atomics with accessors to them.
(Possible implementation:
Type:pair, buffer is twice as long as an atomic, two atomics are stored there. The accessors return the buffer buffer+sizeof(atomic) respectively.)
For lists there is a special value, the empty list. (This can be implemented by setting the pointer to null)
A list is stored as a pair where the first member is a value, the current first element, and the second is a list, the current tail.
An alist is a list of pairs.
Assq and assq-ref is just a find with a predicate on the first element of the pair eq to the value passed in as key. Actually assq-ref is (compose car assq).
What find does is that it recurses on the list. It is something like:
(define (find pred list)
(if (empty list)
#f
(if (pred (car list))
(car list)
(find pred (cdr list)))))
I hope it helps.
This is just a conceptual level, usually a modern implementation has a lot of optimalizations on top of that.
An alist is a list of pairs.
Best regards,
g_bor