Asp Forum - Re: different LOOP results

William James

2/1/2016 5:30:00 PM

Dr. Edmund Weitz wrote:

> The following input
>
> (loop for x in '(1 2 3 4)
> for y upfrom 0
> collect x into temp
> when (oddp y)
> collect temp
> and do (print temp)
> (setq temp nil))
>
> results in
>
> (1 2)
> (3 4)
> ((1 2) (3 4))
>
> with CLISP and LispWorks - which is what I expected. But CMUCL shows
>
> (1 2)
> (1 2 3 4)
> ((1 2 3 4) (1 2 3 4))
>
> instead. Is this a bug or are both behaviors conforming?
>
> (Pardon me if this is trivial, I'm just starting with LOOP...)

Paul Graham:

I consider Loop one of the worst flaws in CL, and an example
to be borne in mind by both macro writers and language designers.

[ In "ANSI Common Lisp", Graham makes the following comments: ]

The loop macro was originally designed to help inexperienced
Lisp users write iterative code. Instead of writing Lisp code,
you express your program in a form meant to resemble English,
and this is then translated into Lisp. Unfortunately, loop is
more like English than its designers ever intended: you can
use it in simple cases without quite understanding how it
works, but to understand it in the abstract is almost
impossible.
....
the ANSI standard does not really give a formal specification
of its behavior.
....
The first thing one notices about the loop macro is that it
has syntax. A loop expression contains not subexpressions but
clauses. The clauses are not delimited by parentheses;
instead, each kind has a distinct syntax. In that, loop
resembles traditional Algol-like languages. But the other
distinctive feature of loop, which makes it as unlike Algol as
Lisp, is that the order in which things happen is only
loosely related to the order in which the clauses occur.
....
For such reasons, the use of loop cannot be recommended.

-----

Dan Weinreb, one of the designers of Common Lisp:

.... the problem with LOOP was that it turned out to be hard to
predict what it would do, when you started using a lot of
different facets of LOOP all together. This is a serious problem
since the whole idea of LOOP was to let you use many facets
together; if you're not doing that, LOOP is overkill.

-----

Barry Margolin, 05 Apr 2001
(http://groups.google.com/group/comp.lang.lisp/msg/8a48ce...)

>(My second rule of thumb concerning LOOP would be the negative of
>Barry Margolin's: The more complex the looping, the more you need/want
>to use LOOP.)

My recommendation is based on seeing many question in the past of the form
"What happens if you use both XXX and YYY in the same LOOP?" The
unfortunate fact is that when we were writing the standard we didn't have
time to nail down all the possible interactions between different LOOP
features, so many of these are not well specified. And even if we did get
it right in the standard, it's likely to be difficult to find them and I
wouldn't trust that all implementors got it right (many of those questions
were probably from implementors, trying to figure out what they were
supposed to do). And even if they all got it right, someone reading your
code may not be able to figure it out.

So, with all those potential problems, my feeling is that if you have to
ask, it's probably better to use something other than LOOP.

-----

Barry Margolin:

> 3. Loop is very powerful, granted, and many people are trying to
> argue that "you can do so much with loop that it's unreadable."
> This is not an argument.

But it is! Because any use of LOOP has the potential to be
unreadable, the reader must read it carefully to verify that
it's just one of the cases that doesn't require careful
reading!

-----

Barry Margolin: (05 Apr 2002 20:57:48 GMT)

This seems like a big change just to clean up the way LOOP is described.
And LOOP will still be a wart, because it will be the only language feature
that uses "per-macro keywords". Providing this interface and giving a name
to them would encourage other macro designers to do something similar, and
we don't want more things like LOOP.

--
[A]n unholy alliance of leftists, capitalists, and Zionist supremacists has
schemed to promote immigration and miscegenation with the deliberate aim of
breeding us out of existence in our own homelands.... [T]he real aim stays the
same: the biggest genocide in human history.... --- Nick Griffin
(https://www.youtube.com/watch?v=K...)

comp.lang.lisp

Re: different LOOP results

William James

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