On Thu, Nov 11, 2010 at 1:21 PM, Eli Barzilay <e...@barzilay.org> wrote: > About a minute ago, Carl Eastlund wrote: >> On Thu, Nov 11, 2010 at 1:14 PM, Eli Barzilay <e...@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. --Carl _________________________________________________ For list-related administrative tasks: http://lists.racket-lang.org/listinfo/users
