Vassil Nikolov  <vnikolov@poboxes.com> wrote:
+---------------
|   rpw3@rigden.engr.sgi.com (Rob Warnock) wrote:
| > +---------------
| > | ...idiom for a half-cons (i.e. a singleton reference)?
| > +---------------
| >
| > Several Scheme implementations have such a notion for a half-cons,
| > conventially called a "box":
...
| Are boxes as efficient as cons cells in terms of speed of access and
| memory use?  (By the latter I mean, does a box take up half the space
| for a cons?)
+---------------

It all depends on the implementation, I'd guess. In MzScheme a box is an
instance of "Scheme_Small_Object", which is a type field [a short, but
most compilers will force that to a PSV] and a pointer-sized value (PSV).
A cons is an instance of "Scheme_Object", which is a type field and
two PSVs. So for MzScheme, a box is 2/3 (or maybe 3/5) the size of a cons.

In an implementation I've been working on [play project], the same is true:
a box is 2/3 the size of a cons.

Chez Scheme (see http://www.cs.indiana.edu/eip/workshop1/lab7/lab73.html)
also has boxes, but I don't know if they're 1/2, 2/3, or 1 times a cons...


-Rob

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Rob Warnock, 8L-855		rpw3@sgi.com
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