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lispcommon-lisp
Roger Nash
Roger Nash

Reputation: 61

Implementation of cons using `list` or `append`

Is there any way by which cons can be implemented in Common LISP using list, append, first, rest etc?

In the following code

(defun my_list (&rest arguments)
   `(,@arguments) ; Line 1
)

What does complete line 1 mean ?

Upvotes: 3

Views: 1605

Answers (4)

Gareth Rees
Gareth Rees

Reputation: 65854

Yes, you could in theory define cons in terms of list and append, like so:

(defun cons (car cdr) (append (list car) cdr))

Upvotes: 1

Alexandre Deschamps
Alexandre Deschamps

Reputation: 1228

I have the answer for the second question:
for the ,:
if my_symbol = 1
`(my_symbol 2 3) = (my_symbol 2 3), but with the ,:

`(,my_symbol 2 3) = (1 2 3)

The , evaluates the next symbol in a ` statement

Now for the @ (which is a symbol, so it needs the , to be activated)

`(,@('a 'b 'c) ('d 'e 'f)) = ('a 'b 'c ('d 'e 'f) )

`(,@('a 'b 'c) ,@('d 'e 'f) ) = ('a 'b 'c 'd 'e 'f)

I hope these examples could help. So the line 1 is simply extracting the arguments from a list and putting them into another one.

Upvotes: 0

Svante
Svante

Reputation: 51501

First question: No, because cons is the building block for list and append, not the other way around. It is like trying to construct a brick out of houses.

Second question: The backquote syntax is explained in the CLHS (http://www.ai.mit.edu/projects/iiip/doc/CommonLISP/HyperSpec/Body/sec_2-4-6.html).

Stylistic comments:

  • It is spelt "Common Lisp".

  • Do not use underscores to separate parts of names, but hyphens: my-list.

  • Do not let parentheses dangle around. Your snippet should be formatted like this:

    (defun my-list (&rest arguments)
  `(,@arguments)) ; Line 1

  • Using the backquote syntax outside of macros is usually not a good idea. In this case, it is completely superfluous:

    (defun my-list (&rest arguments)
  arguments)

Upvotes: 6

Johan Kotlinski
Johan Kotlinski

Reputation: 25729

How about this?

(defun cons (a b) '(a . b))

Or, if you desperately need to use lists...

(defun cons (a b) (first (list '(a . b))))

Upvotes: -1

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