Is there a way to extract all elements of a list in place

Question

Im looking for a way to extract all the elements of a list in common lisp. Like this

[194]> (break-out-of-list '(a b c d))
A
B
C
D

Edit: The usage example I gave was not thought out very well, however I'm still curious if it is possible to break out of a list like in the example above.

Haver

what does 'extract' or 'break out' mean? what does it do? if you want to print list elements to the terminal, map print over them. – Rhombic

With extract I mean the inverse of list. A function where (break-list '((a b c)) ) would return `(a b c) – Haver

The 'inverse'? What is that? In your example you just take the first element. Maybe you mean some kind of 'flatten', where all atoms in a list or its sublists are moved into a single flat list? – Rhombic

Ok, what I want is some construct such that: (some-macro (break-list '(a b c d))) is equivalent to (some-macro 'a 'b 'c 'd). For functions I could use apply as Chris Jester-Young pointed out below. – Haver

Answer 1

What you demonstrate seems to be the question how to get the elements of a list as multiple values:

CL-USER> (values 1 2 3)
1
2
3
CL-USER> (apply #'values '(1 2 3))
1
2
3

See also multiple-value-bind and nth-value in the hyperspec.

Jdavie

Answer 2

While (apply #'values '(1 2 3)) works there is also a function to this called values-list which is used like this:

(values-list '(1 2 3))

And it has the same result.

Dicast

Answer 3

Sure, just use apply:

(defun wraptest (&rest arguments)
  (apply #'test arguments))

This technically doesn't "break out of list"; it simply uses a list's elements as arguments to a function call.

(Disclaimer: I'm a Schemer, not a Common Lisper, and there may be a more-idiomatic way to achieve the same result in CL.)

Thoracotomy

You're right, that works for the example I gave. However the example I gave was a bit flawed so I removed it. I'm mostly curious if a break out of list function or operator exists – Haver

Answer 4

I think you might be looking for this:

http://cl-cookbook.sourceforge.net/macros.html#LtohTOCentry-2

That's mostly all there is to backquote. There are just two extra items to point out. First, if you write ",@e" instead of ",e" then the value of e is spliced into the result. So if v=(oh boy), then `(zap ,@v ,v) evaluates to (zap oh boy (oh boy)). The second occurrence of v is replaced by its value. The first is replaced by the elements of its value. If v had had value (), it would have disappeared entirely: the value of (zap ,@v ,v) would have been (zap ()), which is the same as (zap nil).

Reading your comments:

(some-macro (break-list '(a b c d))) is equivalent to (some-macro 'a 'b 'c 'd)

With this, you could do:

`(some-macro ,@'(a b c d))

and you'd get:

(some-macro a b c d)

Artois

But since I have to use backquote in order to use ,@ I'm still left with the outer list. But yes I'm looking for a equivalent of the,@ operator that can be used without backquote. – Haver

I think you can use it anyway, example underway ;-) – Artois

Wouldn't I have to write a macro that returns `(some-macro ,@'(a b c d)) to use this? – Haver