[racket] member et al.

Two minutes ago, Carl Eastlund wrote:
> On Thu, Nov 11, 2010 at 1:21 PM, Eli Barzilay <eli at barzilay.org> wrote:
> > About a minute ago, Carl Eastlund wrote:
> >> On Thu, Nov 11, 2010 at 1:14 PM, Eli Barzilay <eli at barzilay.org> wrote:
> >> >> I got inspired by the #:when form in `for' loops that flattens
> >> >> nesting, and wrote a `cond*' macro that allows this instead:
> >> >>
> >> >> (cond* [... some stuff ...]
> >> >>         #:with (define m ...)
> >> >>         [(... something about m ...)  (... something using m ...)]
> >> >>         ... more things referring to m ...)
> >> >>
> >> >> It's more general, since it doesn't require encoding the condition
> >> >> you want to test as a truthiness value. Also, the `define' doesn't
> >> >> have to be a `define' - it can be any legal expression.
> >> >
> >> > So, you advocate eliminating `member' etc for predicates (why I
> >> > started with saying that this is much stickier than just adding a
> >> > `member?').  So we started with
> >> >
> >> >  (cdr (assq x alist))
> >> >
> >> > then Scheme upgraded this to
> >> >
> >> >  (cond [(assq x alist) => cdr]
> >> >        [else #f])
> >> >
> >> > and you suggest continuing with
> >> >
> >> >  (cond #:with (define p (assq x alist))
> >> >        [(pair? p) (cdr p)]
> >> >        [else #f])
> >> >
> >> > or, with more well-behaved proper sub lists
> >> >
> >> >  (cond #:with (define p (assq x alist))
> >> >        [(not (eq? p #f)) (cadr p)]
> >> >        [else #f])
> >> >
> >> > (Yes, you can still use `pair?' but that would be ugly for the same
> >> > reason.)
> >> >
> >> > I also wonder how many newbies (or people that just want to type less)
> >> > will fall into traps like
> >> >
> >> >  (if (member? x l)
> >> >    (+ 1 (find x l))
> >> >    0)
> >> >
> >> > My loud "ugh" should be expected now.
> >>
> >> (dict-ref x alist)
> >
> > The above is "an example", substitute appropriately.
> 
> As is mine.  We have a dict library, a sequence library, and loop
> comprehensions for a reason.  Code doing any kind of key/value lookup
> should not have to use cond, car, or cdr to do so.

Either that's a nonsensical argument, or all programmers should now
quit because everything is done.  (In other words, lookups are not the
only use of conditionals.  Proof left as an exercise to the bored
reader.)

-- 
          ((lambda (x) (x x)) (lambda (x) (x x)))          Eli Barzilay:
                    http://barzilay.org/                   Maze is Life!