I fail to see your confusion. You need to read the dictionary very carefully. It's wording is very precise and compact. It is also written in English which is ambiguous. Try to understand what it is saying and the concept it is conveying. Do not second guess it. I studied it for months when I started learning J. Still have a lot to learn from it.
On Sun, Apr 10, 2011 at 4:52 PM, Boyko Bantchev <[email protected]> wrote: > On 10 April 2011 19:55, Raul Miller <[email protected]> wrote: > > But this is not K. If K semantics were intended by the dictionary, > > and they were not explicitly specified, that would be wrong. But why > > should the J dictionary have to assert that J semantics are to be > > used? > > I have already explained that: `applies between' does not specify > direction (L-R or R-L). We know that the order is R-L for an > expression like y1 u y2 u ... u yn, but not for u/y. / is an > operator that has to be defined as strictly and formally as any other. > If / is meant to induce a R-L order of applying a verb, then /'s > definition should better be explicit about that -- which it isn't. > > And K was only mentioned as an evidence that the order of application > induced by u/y is not automatically the one of y1 u y2 u ... u yn: > in principle, it could be any of L-R or R-L, and it is the definition > of / that decides which one it is. There is no implicit semantics of > / -- it has to be defined, whatever the language. > > >> I would have no doubt if the definition said, e.g., > >> `u/y, for y = y1 ... yn and n>1, is equivalent to the expression > >> y1 u y2 u ... u yn'. > > > > Why would you know, in this case -- but not the other case -- that the > > dictionary was not talking of a K expression? > > I don't understand what you mean here. Of course in both cases we > are talking of J expressions. The point is that my example sentence > above *clearly and explicitly* defines the order of evaluation of > u's application within u/y, to be the order of evaluation of the > expression y1 u y2 u ... u yn. Then, since the latter is R-L, the > former is also R-L. Without explicitly sayng that, we simply cannot > be sure about what exactly u/y does -- and this is why J's definition > of / is unclear. > > > I suppose you are arguing that J should reject cases of u/y where y > > contains elements which are not in the domain of u. > > I am not proposing a change to J. I am only making two observations. > First, /'s definition is not entirely clear -- it leaves room for > guessing. Second, /'s actual operation has certain intricacies and > anomalies which I see as a result of / not allowing for an initial > value, in addition to the list, to take part in the computation. > Indeed, Haskell's folds, K's `over', Lisp's reduce, etc., which do > accept an initial value, present no such problems. > ---------------------------------------------------------------------- > For information about J forums see http://www.jsoftware.com/forums.htm > ---------------------------------------------------------------------- For information about J forums see http://www.jsoftware.com/forums.htm
