[racket] member et al.

On Thu, Nov 11, 2010 at 1:29 PM, Eli Barzilay <eli at barzilay.org> wrote:
> 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.)

Is this about conditionals or lookups?  You started with (cdr (assq x
alist)), and I don't see the conditional in that.

I'm not saying programmers should quit using conditionals, I'm saying
they should quit using conditionals for everyday dictionary lookups,
so our measuring stick for conditional syntax should not be how
verbose it makes everyday dictionary lookups.  I don't see how your
original point applies to more esoteric uses of conditionals, because
they never had a (cdr (assq x alist)) form.

--Carl