This result looks perfectly logical to me, under the interpretation that _. means "unknown".
In such a case _. -: _. 0 represents A -: B , not A -: A . But because (unequal) A and B are both unknown they are represented by the same placeholder _. On the other hand -:~ _. is to compare an unknown noun with itself. This is borne out by: A=: _. B=: _. A -: B 0 A -: A 1 -:~ A 1 Sadly for this neat interpretation, you get a different outcome if you replace (-:) with (=). Which suggests that (=) does not support the interpretation of (_.) as "unknown", but "unknowable". But, short of having different symbols, I'm happy to live with an "indeterminate" that covers both interpretations. In spite of Voc's recommendation, in effect, to avoid _. like the plague, I find it really quite useful. Prior to re-implementing TABULA (qv) in J, I needed separate notions of UNDEFINED and INVALID that were propagated to all dependent quantities, and had to model them myself in APL+Win, explicitly and tediously. I'm pleased to discover that _. and _ between them do the job for me. In fact far better than I'd modelled it. On Fri, Jul 1, 2011 at 2:07 PM, Devon McCormick <[email protected]> wrote: > Yes - that's why I'm not sweating this - but shouldn't > > A -: A > > be the same as > > -:~A > > or am I mis-understanding -:~ ? > > Just trying to get my tautologies straight.... > > > On Fri, Jul 1, 2011 at 6:57 AM, Raul Miller <[email protected]> wrote: > >> On Fri, Jul 1, 2011 at 6:52 AM, Devon McCormick <[email protected]> >> wrote: >> > This seems inconsistent: >> > >> > _. -: _. >> > 0 >> > -:~_. >> > 1 >> > >> > Is the latter special-cased? >> > >> > I guess this re-enforces the advice (on >> > http://www.jsoftware.com/jwiki/Essays/Indeterminate) that _. "should be >> > removed...as soon as possible". >> >> _. is inconsistent, by definition. >> >> -- >> Raul >> ---------------------------------------------------------------------- >> For information about J forums see http://www.jsoftware.com/forums.htm >> > > > > -- > Devon McCormick, CFA > ^me^ at acm. > org is my > preferred e-mail > ---------------------------------------------------------------------- > For information about J forums see http://www.jsoftware.com/forums.htm > ---------------------------------------------------------------------- For information about J forums see http://www.jsoftware.com/forums.htm
