[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]
RE: why not "stripes" in: (let ((zebra 'stripes) ... ; strings vs symbo
From: |
Drew Adams |
Subject: |
RE: why not "stripes" in: (let ((zebra 'stripes) ... ; strings vs symbols? |
Date: |
Sun, 29 Dec 2013 08:00:43 -0800 (PST) |
> ATM i read "An Introduction to Programming in Emacs Lisp". In
> the section where the let function is explained in detail the
> author, Robert J. Chassell, uses this "silly" example:
>
> (let ((zebra 'stripes)
> (tiger 'fierce))
> (message "One kind of animal has %s and another is %s."
> zebra tiger))
>
> which when evaluated produces "One kind of animal has stripes and
> another is fierce." as output.
>
> The thing which makes me wonder is why he uses 'stripes instead
> of "stripes" in this example.
Either is OK. They both produce the same effect here.
Use `C-h f format' to see what %s does (versus %S).
> In the output of the message function it makes no difference but
> to me it seems more natural to use strings here since they are
> part of a string in the output...
The beauty of %s is that you can print any Lisp object. For a
symbol, its `symbol-name' is printed with %s.
> I do not really understand how the 'stripes are different
> to "stripes". Isn't 'stripes a notation for the symbol
> stripes? This would mean there is the notion of a symbol which
> is bound to noting?
Yes, and yes. Here it is irrelevant whether the symbol `stripes'
is bound to a value.
A symbol can be used for various things in Lisp. For one thing,
It can act as a variable, having a `symbol-value'. For another,
it can act as a function, having a `symbol-function'.
It can also act as a (rudimentary) OO object, having "slots" or
"attributes", called its symbol "properties". These are stored
on its `symbol-plist', and are accessed using `get' and `put'.
And it need not have a non-nil value for any of these things,
in which case it at least acts as an identity, having a
`symbol-name'.
Unlike strings "stripes" and "stripes", which might be `eq' but
at least are `equal', (if in the same obarray) two symbols
`stripes and `stripes are `eq'. They are the same Lisp object.
For one thing, that generally saves space and makes comparison
quicker.
> Could somebody please enlighten me as to what the differences
> between "stripes" and 'stripes are in which cases which notation
> is more useful/natural?
Natural is in the eye of the beholder. But symbols are powerful
and easy to use in Lisp. They are used a lot.