[racket] Boolean expressions [Was: static variables question]
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
<robby at eecs.northwestern.edu>wrote:
> On Sun, Feb 19, 2012 at 7:10 PM, Neil Toronto <neil.toronto at gmail.com>
> wrote:
> > On 02/19/2012 06:05 AM, Robby Findler wrote:
> >>
> >> On Sun, Feb 19, 2012 at 12:30 AM, Gary Baumgartner<gfb at 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
>
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