How to give a list to cond?

Question

We have a number of clauses (condition body) which we want to give to cond, like so:

(cond (condition1 body1)
      (condition2 body2)
      (condition3 body3))

But imagine that the clauses were first generated via mapcar, which gave the following list:

((condition1 body1)
 (condition2 body2)    
 (condition3 body3))

Let's say that the above list is the content of variable clauses. Now, we would like to pass it to cond, but if we do the following:

`(cond ,clauses)

it results in:

(cond ((condition1 body1)
       (condition2 body2)
       (condition3 body3)))

which does not work, instead of:

(cond (condition1 body1)
      (condition2 body2)
      (condition3 body3))

Is there a way around this problem ?

Answer 1

You want to splice the list into the new form:

`(cond ,@clauses)

See C-hig (elisp) Backquote RET

You can also "splice" an evaluated value into the resulting list, using the special marker ,@. The elements of the spliced list become elements at the same level as the other elements of the resulting list. The equivalent code without using ` is often unreadable. Here are some examples:

     (setq some-list '(2 3))
          ⇒ (2 3)
     (cons 1 (append some-list '(4) some-list))
          ⇒ (1 2 3 4 2 3)
     `(1 ,@some-list 4 ,@some-list)
          ⇒ (1 2 3 4 2 3)

     (setq list '(hack foo bar))
          ⇒ (hack foo bar)
     (cons 'use
       (cons 'the
         (cons 'words (append (cdr list) '(as elements)))))
          ⇒ (use the words foo bar as elements)
     `(use the words ,@(cdr list) as elements)
          ⇒ (use the words foo bar as elements)