Re: alist and assq

["Gábor Boskovits" <boskovits@gmail.com>]

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Hello,

Freeman Gilmore <freeman.gilmore@gmail.com> 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

>

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