[racket] member et al.

Neil had an example upthread.

Robby

On Thu, Nov 11, 2010 at 12:36 PM, Carl Eastlund <cce at ccs.neu.edu> wrote:
> 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
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