Question about quoted list syntax...

[jdiamond]

I noticed that an expression like:
(cons 1 '(2 3))
is illegal in both LISP and ACL2. In both environments, they see the
' as a strange extra parameter.

Some legal (accepted versions) are:
(cons 1 (list 2 3))
(cons 1 (quote (2 3))

but not

(cons 1 ('(2 3)))

even though
'(2 3)
by itself is accepted.

I thought a quoted list was a valid term, but it seems like it is not
allowed as an argument.

Any comments as to the scope where you can use a quoted list?
Thanks,
- Jeff

[Sandip Ray]

Hi,

(cons 1 '(2 3)) is a well-defined thing and it stands for (cons '1
(cons '2 (cons '3 nil))) which actually evaluates to the object (1
. (2 . (3 . nil))) that is abbreviated as the list (1 2 3).

I believe the mistake you made is with an extra parenthesis. You
wrote (cons 1 ('(2 3))), which is indeed illegal. This is because
('(2 3)) does not mean anything. If we start with an open parenthesis
without a quote in front then what is expected is a function symbol.
But this is not provided. It's worth noting that '(2 3) is a way of
writing (quote 2 3), but ('(2 3)) is not. If instead you write
(cons 1 '(2 3)), that is legal and accepted.

Let me know if anything is still unclear.

-- Sandip

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[jdiamond]

As mentioned in another email, I was having false negatives due to a
character translation error when I cut and pasted text - my quotes got
marred.

But here is a VERY KEY PRINCIPLE you need to correctly evaluate this
stuff:
Expand Macros BEFORE abbreviations.

This is something taken for granted by experienced LISP people, but
it's the only way that you can take a definition like this:
'(a . b) => (cons 'a 'b)
and apply it logically to a situation like this:
'(a b) => ???
Apply the list defmacro first:
'(cons a (cons b nil))
and only then apply the abbreviation revursively:
=> (cons 'a '(cons b nil)) => (cons 'a (cons 'b 'nil))

So far, it looks like defmacro is very similar to the C preprocessor -
> it happens before any interpretation takes place.