Oh, good point! I've adjusted implies to just take two arguments. Meanwhile, I've also added a binary xor. It returns the non-#f argument when exactly one of the arguments is #f and otherwise returns #f. Comments welcome.
Robby On Mon, Feb 20, 2012 at 5:28 AM, Carl Eastlund <c...@ccs.neu.edu> wrote: > If (implies a b ... z) is equivalent to (implies a (implies b ... z)), then > it is also equivalent to (implies (and a b ...) z). In which case, the > 1-ary case should be clear: just return z. In truth, it is not really > necessary to have n-ary implies if you're willing to nest the (and ...) > explicitly. Given that Scheme/Racket n-ary operators tend to have the "..." > last, with occasional exceptions like list* and apply, restricting implies > to 2 arguments might avoid cases where (implies a b c d e) is confusing to > read. (implies (and a b c d) e) is clearer. > > Carl Eastlund > > On Sun, Feb 19, 2012 at 8:16 PM, Robby Findler <ro...@eecs.northwestern.edu> > wrote: >> >> On Sun, Feb 19, 2012 at 7:10 PM, Neil Toronto <neil.toro...@gmail.com> >> wrote: >> > On 02/19/2012 06:05 AM, Robby Findler wrote: >> >> >> >> On Sun, Feb 19, 2012 at 12:30 AM, Gary Baumgartner<g...@cs.toronto.edu> >> >> wrote: >> >>> >> >>> On a more productive note: in Racket code I define and use 'implies' a >> >>> lot, >> >>> often conjoined, for predicates. It's mainly of declarative value, >> >>> which >> >>> is >> >>> perhaps why it's uncommon in implementation despite how common it is >> >>> in >> >>> specification. And for boolean expressions in general I also use >> >>> 'neither'. >> >>> Are these something that others [would] use and so could be added to >> >>> Racket's >> >>> library? >> >> >> >> >> >> Seems to me adding implies, nand, and nor to racket/bool is a good >> >> idea. >> >> >> >> Let me know if you disagree (and if you disagree after I've already >> >> committed, it is a simple thing to drop the commit or change it). >> > >> > >> > I've occasionally written my own `implies', but never make it shortcut >> > like >> > `and' and `or'. Speaking of which, would it handle more than two >> > arguments? >> > One? Zero? What kind of associativity would be appropriate? >> >> The version I checked in shortcuts in an implication-appropriate way >> (if the first argument is #f). I think the convention (due to >> functions and currying) is pretty well established that it associates >> to the right. So that's what the version I checked in does for the 3+ >> argument case. I didn't see something reasonable for the 0 or 2-ary >> case so it doesn't handle them. >> >> > Bwahaha. >> >> :) >> >> I'm still pondering those questions for xor. :) >> >> Robby >> >> ____________________ >> Racket Users list: >> http://lists.racket-lang.org/users > > ____________________ Racket Users list: http://lists.racket-lang.org/users
