CODEWITHSUNDEEP
lispcommon-lisp
Gregorio Di Stefano
Gregorio Di Stefano

Reputation: 1203

Can 'cond' fully replace 'if' statements?

I know 'cond' is based off 'if', but can 'cond' do everything 'if' can do? Thanks

Upvotes: 3

Views: 3030

Answers (3)

Matthias Benkard
Matthias Benkard

Reputation: 15759

Yes. Any conditional

(if x y z)

using if can be transformed into an equivalent conditional using cond:

(cond (x y)
      (t z))

For example, consider the following:

(if (= 1 2) (format t "crazy~%") (format t "sane~%"))

The above code can be transformed into:

(cond ((= 1 2) (format t "crazy~%"))
      (t (format t "sane~%")))

Upvotes: 8

nwc
nwc

Reputation: 21

Depending on which lisp you're dealing with, if and cond are definitely not always interchangeable. Here's a test to find out. The results I give are from MIT's (2014) Scheme interpreter.

Test

We may design a new-if method made using cond:

(define (new-if predicate then-clause else-clause)
   (cond (predicate then-clause)
         (else  else-clause)))

We may try the following with the old-if and the new-if:

(define (recursive-func a b)
    (if (= a 0) b
        (recursive-func a b)))

(recursive-func 0 1)

=> 1

And with new-if:

(define (recursive-func a b)
    (new-if (= a 0) b
         (recursive-func a b)))

(recursive-func 0 1)

=> ;Aborting!: maximum recursion depth exceeded

Upvotes: 2

Will Hartung
Will Hartung

Reputation: 118601

"if" may well be implemented as a macro around "cond". "cond" is the actual "primitive" here, not "if".

Upvotes: 5

Related Questions