rbe@yrloc.ipsa.reuter.COM (Robert Bernecky) (04/29/91)
A recent posting suggests that the rank adverb and the mix[.1] (etc)
sort of axis brackets are equivalent.
It is certainly true that APL2 expressions with axis brackets can
(or ISO APL with axis brackets) can perform many of the same operations
as those expressed with the rank adverb. FOr a few examples of this,
you can read my APL88 paper (ACM SIGAPL Quote Quad, vol.18, no. 2).
However, there are two problems with axis brackets which make them inferior
to the rank adverb:
a. The syntax is anomalous: The brackets do not conform to the syntax
of functions (verbs), nor of operators( Adverbs), nor any other
APL object except brackets. This complicates syntax analysis, and
makes good fodder for religious jihads.
b. The SEMANTICS of bracket axis notation is NOT defined. That is,
there is NO place you can go and read a definition of what they
mean. Like the caterpillar, axis brackets mean exactly what the
implementor meant and nothing more. \cite{alice}
The meaning of brackets is different for EACH function to which
they may apply, and the only way to determine that meaning is to
have a reference manual by your side. FOr example (I am doing
this from memory, so please bear with me if I screw up), the
APL22 definition of ravel with axis allows specification of a set
of axes in brackets, but requires that the set of axes be contiguous
numbers. Other functions (perhaps mix or disclose?) allow any
set, or fractional sets, etc.
This may not sound important at first glance, but consider the
difference between the expressions:
apl2: X f[k] Y
J: X f"k Y
If you do not know the definition of f (it might be user-defined!),
you cannot tell anything about the action of brackets in APL2.
In J, by contrast, it has a CLEAR and CONSISTENT definition, which
applies to user-defined verbs in exactly the same way as it does
to primitive verbs.
Why is this difference important?
a. It makes the language easier to learn and use. You don't have to
keep running to a reference manual to learn when you can use brackets,
and when you have to split/apply/mix with brackets instead.
b. There is LESS Language to learn: This makes it easier to teach, and
easier to implement.
c. By removing the special cases which axis brackets represent, we end
up with a language with simpler syntax, fewer chances to introduce
bugs, and thereby create more reliable programs.
Bob
Robert Bernecky rbe@yrloc.ipsa.reuter.com bernecky@itrchq.itrc.on.ca
Snake Island Research Inc (416) 368-6944 FAX: (416) 360-4694
18 Fifth Street, Ward's Island
Toronto, Ontario M5J 2B9
CanadaEdward_M_Cherlin@cup.portal.com (05/06/91)
To me, the real problem with anomalous APL constructs is that they are not functional. This means that they cannot be used in operator expressions, in particular, and suffer from all of the deficiencies that functional programming is intended to remedy, in general. See my forthcoming paper, "Pure Functions in APL and J", APL91 conference, Stanford, Aug. 1991, and my articles in recent issues of APL News on J. I include brackets (axis, subscript), diamond, and QuadFX as anomalies, following Iverson. These are the features he felt most needed fixing, and he has fixed them in J.
mgf@ipsaint.ipsa.reuter.COM (Gfeller, Martin) (05/07/91)
-----------Message forwarded from IPSA Mailbox------------- no. 5166696 filed 18.58.03 mon 6 may 1991 from mgf to uclapl subj Re: mixing it up: The role of fractional axis brackets ref 5166626 Branch and assignment arrows are not functional as well. J maps them both to one, and calls it copula. I don't think copulas can be modified by adverbs, so they're not strictly functional in J either. But this is a hard area. /Martin ----------------------------------------------------------- This posting is forwarded from an internal Reuters mailbox. No statement or opinion contained herein should be taken as being Reuters policy, or even as being approved by Reuters, in any way.
ljdickey@watmath.waterloo.edu (L.J.Dickey) (05/07/91)
Date: Tue, 7 May 1991 13:52:26 GMT Lines: 26 In article <1991May6.204056.26134@yrloc.ipsa.reuter.COM> mgf@ipsaint.ipsa.reuter.COM (Gfeller, Martin) writes: >no. 5166696 filed 18.58.03 mon 6 may 1991 >from mgf >subj Re: mixing it up: The role of fractional axis brackets > >Branch and assignment arrows are not functional as well. J maps them >both to one, and calls it copula. I don't think copulas can be >modified by adverbs, so they're not strictly functional in J either. >But this is a hard area. /Martin I dont understand this, on two counts. (1) There are two copulas, not one: "=." and "=:" . (2) Order of execution of lines in a function is controlled by "suite", ($.), and the suite may be modified within a function by doing an assignment. I wonder if this is what was meant. It is not quite right to say that branch is mapped to assignment.