A critic of Guido's blog on Python's lambda

[M Jared Finder]

Chris Uppal wrote:

> E.g. can you add three-way comparisons (less-than, same-as, greater-than to,
> say, Python with corresponding three-way conditional control structures to
> supplement "if" etc ?  Are they on a semantic and syntactic par with the
> existing ones ?  In Smalltalk that is trivial (too trivial to be particularly
> interesting, even), and I presume the same must be true of Lisp (though I
> suspect you might be forced to use macros).

As an illustration, here's the definition and usage of such a numeric-if 
in Lisp.

Using raw lambdas, it's ugly, but doable:

(defun fnumeric-if (value lt-body eq-body gt-body)
   (cond ((< value 0) (funcall lt-body))
         ((= value 0) (funcall eq-body))
         ((> value 0) (funcall gt-body))))

(fnumeric-if (- a b)
   (lambda () (print "a < b"))
   (lambda () (print "a = b"))
   (lambda () (print "a > b")))

A macro helps clean that up and make it look prettier:

(defmacro numeric-if (value lt-body eq-body gt-body)
   `(fnumeric-if ,value
                 (lambda () ,lt-body)
                 (lambda () ,eq-body)
                 (lambda () ,gt-body)))

(numeric-if (- a b)
   (print "a < b")
   (print "a = b")
   (print "a > b"))

   -- MJF