On Fri, Oct 28, 2011 at 12:53 PM, Robby Findler <ro...@eecs.northwestern.edu> wrote: > On Fri, Oct 28, 2011 at 11:44 AM, Carl Eastlund <c...@ccs.neu.edu> wrote: >> On Fri, Oct 28, 2011 at 12:07 PM, John Clements >> <cleme...@brinckerhoff.org> wrote: >>> >>> On Oct 28, 2011, at 8:12 AM, Joe Marshall wrote: >>> >>>> On Wed, Oct 26, 2011 at 8:32 PM, Dan Grossman <d...@cs.washington.edu> >>>> wrote: >>>>> Very minor point, but is there a rationale beyond historical precedent >>>>> for + and * to allow any number of arguments but, =, <=, <, >, >= to >>>>> require at least two arguments? >>>> >>>> 0 is the additive identity. 1 is the multiplicative identity. >>>> What is the equality identity? >>> >>> No, I don't buy that. operators in \alpha X \alpha -> \beta can never have >>> identities, but that doesn't mean they can't be generalized. >>> >>> I can definitely imagine that you would choose to disallow unary use of >>> comparison operations to prevent a certain class of programming errors, but >>> it seems pretty clear to me that the generalization of, e.g., <= is "is >>> every sequential pair of items in the argument list related by the given >>> operator." >>> >>> Am I missing something here? >>> >>> John >> >> Furthermore, that generalization is useful, as it makes (apply <= xs) >> into a simple implementation of "is xs monotonically non-decreasing?", >> just as (apply + xs) implements "the sum of the elements of xs". If >> <= must accept 2 or more arguments (or even 1 or more), that does not >> work for all lists. Personally, I'd prefer if <= and friends were >> generalized. It seems more in tune with Racket's permissive Scheme >> heritage -- if append accepts "improper" lists, + mixes precise >> numbers with floating point, and all values act as booleans, why can't >> <= accept 0 or 1 arguments? > > Does this rationale also suggest that it is fine that (cdr '()) = '()? > > Robby
No. I don't see any sense in which that's a generalization of cdr. In fact it's quite non-uniform; for instance, applying cdr to a non-empty list produces a list one shorter. If we want to extend cdr to length 0 lists, we need to somehow produce a list of length -1. We restrict cdr from empty lists precisely because it can't be generalized. Or at least that's how I see it. --Carl _________________________________________________ For list-related administrative tasks: http://lists.racket-lang.org/listinfo/users
