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From: johnl@haddock.UUCP
Newsgroups: net.lang
Subject: Re: APL myths - (nf)
Message-ID: <196@haddock.UUCP>
Date: Tue, 26-Jun-84 23:38:21 EDT
Article-I.D.: haddock.196
Posted: Tue Jun 26 23:38:21 1984
Date-Received: Sat, 30-Jun-84 02:24:07 EDT
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#R:decwrl:-198200:haddock:12300006:000:1625
haddock!johnl    Jun 26 11:04:00 1984

I also used to think that APL was unreadable, but discovered that I was
going about it the wrong way.  APL turns out to be quite readable once
you understand how to read it.  Certainly, if you try to read an APL
program the same way you'd read a C or Fortran program, you'd get quite
frustrated.

First, it's OK that it takes longer to read a line of APL that it takes
to read a line of Fortran, because a line of APL does more than a line of
Fortran.

Then, you have to read by idioms.  In Fortran, you can look at this and tell
pretty quickly what it does because you've seen it 1000 times before:

	xmax = 0
	do 100 i = 1, n
	   if(a(i) .gt. xmax) xmax = a(i)
100     continue

In APL, an analogous thing would be

	XMAX gets ceiling / A

which is as easy to read.  You read larger expressions by recognizing
familiar "idioms" (a term coined by A. J. Perlis, who has published long
lists of his favorite APL idioms.)  For example, "X iota X" is an idiom
used in finding or removing duplicates in a vector where the Nth element
is N if the corresponding entry in X is different from all the previous
ones and less than N if not, and "iota rho X" is the numbers from 1 to
the size of X, i.e. the subscripts of all of the entries in X.  So:

	((iota rho X) = X iota X) / X

turns out to be an expression that is X with the duplicates removed.  But
you look at that, recognize "X iota X" and "iota rho X" as familiar
idioms, and you're well on your way to understanding it.  After a while
you learn to recognize such larger idioms, too.

John Levine, ima!johnl

PS:  I still think that APL's I/O, in whatever version, stinks.