Re: lisp style question

[RG <rNOSPAMon@flownet.com>]

In article 
<6e2fe51c-ff4c-429f-b221-a3cbe23e958f@n2g2000pre.googlegroups.com>,
 Katalin Sinkov <lispstylist@gmail.com> wrote:

> On Dec 2, 12:50 am, "Frode V. Fjeld" <fr...@netfonds.no> wrote:
> > Katalin Sinkov <lispstyl...@gmail.com> writes:
> > > In the {} world I would return a small table like
> >
> > > width   1
> > > height  2
> > > weight  3
> >
> > Typically in Lisp you'd return either a property or association list.
> >
> > I.e: (WIDTH 1 HEIGHT 2 WEIGHT 3) with accessor GETF,
> >
> > or ((WIDTH . 1) (HEIGHT . 2) (WEIGHT . 3)) with accessor ASSOC.
> >
> > --
> > Frode V. Fjeld
> 
> Of all the four or five replies, I found yours most helpful although
> brief. This is perhaps due to me being a beginner, although the
> replies seem very promising and I am desirous of understanding them. I
> have just read the paper by McCarthy and the micro manual.

There's another solution that doesn't seem to have been mentioned yet:

? (defstruct thing width height weight)
THING
? (setf thing (make-thing :width 1 :height 2 :weight 3))
#S(THING :WIDTH 1 :HEIGHT 2 :WEIGHT 3)
? (thing-height thing)
2
? (slot-value thing 'height)
2

> what is an "assoc list" and "a property list" and their difference ?

The difference is purely one of convention.  Both are simple 
arrangements of cons cells, but an association list (also called an 
assoc list of an a-list) looks like this:

((key . value) (key . value) ...)

while a property list (also called a p-list) looks like this:

(key value key value ...)

These are not the only ways to create associative maps using cons cells.  
For example, there is a little-used convention that I call a D-List (for 
"dissociated association list") that looks like this:

((key key ...) value value ...)

The advantage of a D-List is that multiple D-List can share the same set 
of keys so if you have a lot of associative maps with the same keys this 
can be a big performance win.  There are lots of other efficiency hacks 
you can do on D-Lists that you can't do on A-Lists or P-Lists, but 
that's probably more advanced than you want to get right now.

> what is "setf" and how to write it in terms of the elementary
> functions, car/cdr/cons/quote/cond/atom/eq ?

SETF is a macro, not a function, and a particularly complicated one.  
You should read up on macros in general before trying to understand how 
SETF is implemented.

> how to conveniently costruct the list that goes with getf ?

You can't construct anything with GETF.  GETF is an accessor.

rg