[racket] Cond function with test-expr = then-body works but is it wise?

From: Laurent (laurent.orseau at gmail.com)
Date: Fri May 24 12:48:29 EDT 2013

Just for fun, if you have to use the value of the tests in several bodies,
you may also define your own macro:

#lang racket

(define-syntax-rule (cond/let id [test body ...] ...)
  (cond [test => (λ(id) body ...)]

(let ([x "123a" #;"123"])
  (cond/let y
            [(string->number x) (* y 10)]
            [(regexp-match #px"\\da" x) y]
            [#t y]))

Of course, it's just a proof of concept, it doesn't handle `else' and
one-expression clauses, it doesn't verify that id is an identifier, but you
should get the idea.


On Fri, May 24, 2013 at 6:19 PM, Eli Barzilay <eli at barzilay.org> wrote:

> 10 minutes ago, Don Green wrote:
> > It is possible to create a cond test-expr that also serves as the
> then-body,
> > however, this means that the expression would be executed twice.
> >
> > This seems to be ill advised in principle.
> `cond' can have one-expression clauses that serve both purposes:
>   -> (cond [(printf "123\n")])
>   123
> It's convenient to use in rare cases, but I usually try to avoid the
> temptation since it feels wartish...
> > Question: If it is ill-advised to create a cond test-expression that also
> > serves as the then body, then what function is recommend?
> >
> > I am aware of the (cdr (or ... form.
> (This looks like some assq use, which should benefit from the above.)
> --
>           ((lambda (x) (x x)) (lambda (x) (x x)))          Eli Barzilay:
>                     http://barzilay.org/                   Maze is Life!
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