Re: identity function with an echo side effect

[TheFlyingDutchman <zzbbaadd@aol.com>]

> (cdr (print (cdr (print (cdr (print (a b c d)))))))    C-x C-e
>
> gives this error if I remove the quote. why ? I get errors if I remove
> all prints. This means if the first cdr required quoted list, then the
> rest must also require it. Hence, an implicit quotation might be
> occurring as in setq ?

Each cdr requires the same thing - a list object. The first cdr does
not receive "(a b c d)" as its argument, it receives that list literal
turned into its in-memory representation as a  list object. Each cdr
returns a list object (the internal format - not a string literal) to
the next cdr.

The Emacs Lisp interpreter has to turn the (a b c d) "list literal"
into a list object before passing it to the first cdr function.
However, Emacs Lisp has a problem that most languages do not have. A
list literal looks exactly the same as a function call. e.g.:

(min a b)   => Emacs Lisp function call   OR Emacs Lisp list literal

The above could be a list, with symbols min, a and b. Or it could be a
function call of min with parameters a and b. In contrast, the Python
interpreter would not have a problem differentiating between the two
because there is different syntax for each:

min(a ,b)  => Python function call   [min, a, b]   =>  Python list
literal

So how does the Emacs Lisp interpreter differentiate between a
function call and a list literal? It doesn't. It assumes everything is
a function call - or similar expression (macro, special form). If you
are actually writing a list literal and not an expression to evaluate,
you have to explicitly tell the Emacs Lisp interpreter not to evaluate
it as an expression, but to just store it as a list object.

so (a b c d) to Emacs Lisp is a call to the function of symbol a, and
it should pass b, c and d as parameters.   '(a b c d)  or (quote (a b
c d)) tells Emacs Lisp not to try and execute any function stored in a
- just store the whole thing as a list internally.