[racket] member et al.

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.

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