RE: orthogonality and generalized references (was Re: Zen of Python)

["Todd Proebsting" <address@hidden>]

Thank you, Guy, for omitting the "Why didn't you enumerate all *four*
possibilities?" that others less kind might have asked pointedly.  I
will enumerate all four, leaving the two you discuss below adjacent.

    WITH GOAL-DIRECTION            WITHOUT GOAL-DIRECTION
 dict["foo"] :=  dict["bar"]      dict["foo"] :=  dict["bar"]

 dict["foo"] := \dict["bar"]      tmp := dict["bar"]
                                  if tmp is not null then
                                     dict["foo"] := tmp

/dict["foo"] :=  dict["bar"]      lval := lvalueOf(dict["foo"])
                                  if rvalueOf(lval) is null then
                                     tmp := dict["bar"]
                                     rvalueOf(lval) := tmp

/dict["foo"] := \dict["bar"]      lval := lvalueOf(dict["foo"])
                                  if rvalueOf(lval) is null then
                                     tmp := dict["bar"]
                                     if tmp is not null then
                                        rvalueOf(lval) := tmp


To me, the difference between the last two is *less* clear when written
without the unary operators (right column) because the second null test
is lost in a sea of characters.  With the unary operators and GD (left
column), it's clear that the difference is whether or not a null value
can be assigned to the (previously null) left-hand side.  I consider
this an example of why concise/succinct/terse languages can be clearer
than less concise languages.  It's a balance.

Of course, what makes Guy's example "subtle" is that a little reasoning
about the values involved indicates that the two are behaviorally
indistinguishable.

Cheers,
Todd 

-----Original Message-----
From: Guy Steele - Sun Microsystems Labs [mailto:Guy.Steele@sun.com] 
Sent: Tuesday, May 28, 2002 2:28 PM
To: Guy.Steele@sun.com; kragen@pobox.com; ll1-discuss@ai.mit.edu; Todd
Proebsting
Subject: RE: orthogonality and generalized references (was Re: Zen of
Python)


Excellent analysis, Todd; thank you.  Now let us contemplate the even
more subtle contrast, in Icon, between

  /dict["foo"] := \dict["bar"]

and

  /dict["foo"] := dict["bar"]

Gee1  This is almost as much fun as nested backquotes!  :-)

--Guy