Hello,
Freeman Gilmore <[email protected]> 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.
> Thank you, ƒg
>
Best regards,
g_bor
>
