CODEWITHSUNDEEP
common-lisp
marielcomad
marielcomad

Reputation: 13

COND clause with test only

When COND is given only one test clause and nothing else at all it always returns the test result: 

 CL-USER> (cond (t))
    T
 CL-USER>  (cond ((> 5 10)))
    NIL

Isn't COND just a way to write IF statements? This doesn't hold for this as when rewriting COND with test only:

    CL-USER> (if (> 5 1))    
    error while parsing arguments to DESTRUCTURING-BIND:
  too few elements in
    ((> 5 1))
  to satisfy lambda list
    (SB-IMPL::TEST SB-IMPL::THEN &OPTIONAL SB-IMPL::ELSE):
  between 2 and 3 expected, but got 1

If it is this way then how does COND exactly transform every clause into IF version?

Upvotes: 1

Views: 171

Answers (1)

Joshua Taylor
Joshua Taylor

Reputation: 85853

When COND is given only one test clause and nothing else at all it always returns the test result:

That's right. According to the HyperSpec entry on cond:

results---the values of the forms in the first clause whose test-form yields true, or the primary value of the test-form if there are no forms in that clause, or else nil if no test-form yields true.

Isn't COND just a way to write IF statements?

Well, cond is declared to be a macro, and if to be a special operator, so you can think of it that way, although the expansion of cond isn't defined specifically. But if is defined with syntax that requires a then-part, whereas cond isn't. Here's what a cond clause with no forms expands to (in SBCL):

CL-USER> (pprint (macroexpand '(cond ((+ 2 3)))))

(LET ((#:G1013 (+ 2 3)))
  (IF #:G1013
      #:G1013
      (COND)))

The value of the form is saved in a value and then used as the then-part. When there are forms in the clause, they're wrapped in a progn (which returns the value of its last form):

CL-USER> (pprint (macroexpand 
                  '(cond
                    ((= 2 3) 42 78)
                    ((+ 2 3))
                    ((= 4 5) (print 'something-else)))))

(IF (= 2 3)
    (PROGN 42 78)
    (COND ((+ 2 3)) ((= 4 5) (PRINT 'SOMETHING-ELSE))))

Upvotes: 2

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