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From: Maciek Godek
Subject: References/locations
Date: Fri, 8 Aug 2008 03:50:33 +0200

it's me again, asking silly questions.
This time I would like to know if there's
a simple way to store a reference to
a variable in another variable -- say,
an element of vector or hash table.

I imagine this could look like this:
(define v #(1 2 3))
(define v1 (vector-location v 1))
: 2
(set! v1 10)
: #(1 10 3)

I've tried to do it using a "procedure with
setter", but the problem is that set! doesn't
evaluate its first argument (as long as it's a
symbol), so I'd have to wrap everything
up in macros to obtain:
(set! (vector-location v 1) 10)

Besides I think that the names "hash-ref"
and "vector-ref" are confusing, since they
don't return references, but values (therefore
the names like "vector-get" or "hash-get" would
be more apropreate)

I also wonder if there's any point for allowing
locations to any sorts of variables (similar to
pointers in C or pointers to C++), that is,
(define x 10)
(define y (location x))
(set! y 20)
: 20

I expect that this idea isn't new in the world of
lisp, but I didn't find any discussion on the web.
There are certainly some issues related with locations:
- they require special behavior of set! and define:
(set! y 20) could work in two different ways,
depending on whether y is a location or not
- what about "locations of locations"?
- how would it affect performance?
- what about multiple threads?


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