From Newsgroup: comp.lang.lisp

B. Pym wrote:

Jochen wrote:

This ...

CL-USER 1 > (loop for i for j in '(a b c) collect (cons i j))

((0 . A) (1 . B) (2 . C))

works perfectly on LispWorks, but not with SBCL

FOR is an unknown keyword in FOR or AS clause in LOOP.

I found a way around it 'for i from 0' but am not sure if there is another smarter way to achieve this.

I think the canonical way would be

(loop for i upfrom 0
for j in '(a b c)
collect (cons i j))

newLISP

(map (curry list $idx) '(a b c))

((0 a) (1 b) (2 c))

Gauche Scheme

(let ((i (makeigen)) (col (makelbag)))
(dolist (j '(a b c)) (col (cons (i) j)))
(col))

((0 . a) (1 . b) (2 . c))

Given:

;; Make an integer generator.
(define (makeigen :optional (start 0) (step #f) (m #f))
(let ((i start)
(s step)
(m m)) ;; mod
(if m
(lambda () (begin0 (mod i m) (inc! i s)))
(if s
(lambda () (begin0 i (inc! i s)))
(lambda () (begin0 i (inc! i)))))))

(define (makelbag :optional (raw #f) (pass-through #t))
(let ((bag '()))
(lambda args
(if (null? args)
(if raw bag (reverse bag))
(if pass-through
(begin0 (car args) (push! bag (car args)))
(push! bag (car args)))))))
--
[T]he problem is that lispniks are as cultish as any other devout group and basically fall down frothing at the mouth if they see [heterodoxy].
--- Kenny Tilton
The good news is, it's not Lisp that sucks, but Common Lisp. --- Paul Graham --- Synchronet 3.21a-Linux NewsLink 1.2

From Newsgroup: comp.lang.lisp

Jochen wrote:

This ...

CL-USER 1 > (loop for i for j in '(a b c) collect (cons i j))

((0 . A) (1 . B) (2 . C))

works perfectly on LispWorks, but not with SBCL

FOR is an unknown keyword in FOR or AS clause in LOOP.

I found a way around it 'for i from 0' but am not sure if there is
another smarter way to achieve this.

I think the canonical way would be

(loop for i upfrom 0
for j in '(a b c)
collect (cons i j))

When variables iterate independently, they can be joined by
rCy:andrCO or rCyandrCO in place of the subsequent rCyforrCO.

For example, (colons added for emphasis):

(loop
:for i :upfrom 0 :and j :in '(a b c)
:collect (cons i j))

((0 . A) (1 . B) (2 . C))

Both control when the loop ends, so the first iterating
expression can be the constraint on when to stop...

(loop
for i below 2 :and j in '(a b c)
collect (cons i j))

((0 . A) (1 . B))

...or the second iterating expression can be the constraint
on when to stop:

(loop
for i below 1000 :and j in '(a b c)
collect (cons i j))

((0 . A) (1 . B) (2 . C))

(loop
for i downfrom 9 :and j in '(a b c)
collect (cons i j))

((9 . A) (8 . B) (7 . C))

See http://www.ai.mit.edu/projects/iiip/doc/CommonLISP/HyperSpec/Body/sec_6-1-2-1.html

"If multiple iteration clauses are used to control
iteration, variable initialization and stepping[1] occur
sequentially by default. The AND construct can be used to
connect two or more iteration clauses when sequential
binding and stepping[1] are not necessary. The iteration
behavior of clauses joined by AND is analogous to the
behavior of the macro DO with respect to DO*."
--
The lyf so short, the craft so long to lerne.
- Geoffrey Chaucer, The Parliament of Birds.
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From Newsgroup: comp.lang.lisp

B. Pym wrote:

Jochen wrote:

This ...

CL-USER 1 > (loop for i for j in '(a b c) collect (cons i j))

((0 . A) (1 . B) (2 . C))

works perfectly on LispWorks, but not with SBCL

FOR is an unknown keyword in FOR or AS clause in LOOP.

I found a way around it 'for i from 0' but am not sure if there is another smarter way to achieve this.

I think the canonical way would be

(loop for i upfrom 0
for j in '(a b c)
collect (cons i j))

Gauche Scheme

"!" is somewhat like "do" except that it is optimized
for brevity. So this is shorter than the "loop" solution.

(! (r cons (cons i j)
i 0 +
j :in '(a b c))
(not j) @)

((0 . a) (1 . b) (2 . c))

Given:

(define-syntax !-aux
(syntax-rules (<> @ + - cons cdr :in :across :if ! )
[(_ (:if bool z ...) ((v i u) seen ...) stuff ...)
(!-aux (z ...)
((v i (if bool u v)) seen ...) stuff ...) ]
[(_ (x :in lst z ...) seen (lets ...) stuff ...)
(!-aux (x (and (pair? xs)(pop! xs)) <> z ...)
seen
(lets ... (xs lst)) stuff ...) ]
[(_ (x :across vec z ...) seen (lets ...) stuff ...)
(!-aux (x (and (< i (vector-length v))
(begin0 (vector-ref v i) (inc! i))) <>
z ...)
seen (lets ... (v vec) (i 0)) stuff ...) ]
[(_ (a b <> z ...) (seen ...) stuff ...)
(!-aux (z ...) (seen ... (a b b)) stuff ...) ]
[(_ (a b + z ...) (seen ...) stuff ...)
(!-aux (z ...) (seen ... (a b (+ 1 a))) stuff ...) ]
[(_ (a + n z ...) (seen ...) stuff ...)
(!-aux (z ...) (seen ... (a 0 (+ n a))) stuff ...) ]
[(_ (a b - z ...) (seen ...) stuff ...)
(!-aux (z ...) (seen ... (a b (- a 1))) stuff ...) ]
[(_ (a cons b z ...) (seen ...) stuff ...)
(!-aux (z ...) (seen ... (a '() (cons b a))) stuff ...) ]
[(_ (a b cdr z ...) (seen ...) stuff ...)
(!-aux (z ...) (seen ... (a b (cdr a))) stuff ...) ]
[(_ (a b c z ...) (seen ...) stuff ...)
(!-aux (z ...) (seen ... (a b c)) stuff ...) ]
[(_ (a b) (seen ...) stuff ...)
(!-aux () (seen ... (a b)) stuff ...) ]
[(_ (a) (seen ...) stuff ...)
(!-aux () (seen ... (a '())) stuff ...) ]
;;
[(_ () seen lets a b c ! action ...)
(!-aux () seen lets (a b c) #t (action ...)) ]
[(_ () seen lets a b ! action ...)
(!-aux () seen lets (a b) #t (action ...)) ]
[(_ () seen lets a ! action ...)
(!-aux () seen lets a #t (action ...)) ]
;;
[(_ () ((a b c) z ...) lets bool)
(!-aux () ((a b c) z ...) lets bool a) ]
[(_ () ((a b c) z ...) lets bool @)
(!-aux () ((a b c) z ...) lets bool (reverse a)) ]
[(_ () seen lets bool @ result stuff ...)
(!-aux () seen lets bool (reverse result) stuff ...) ]
[(_ () seen lets bool (what @ x z ...) stuff ...)
(!-aux () seen lets bool (what (reverse x) z ...) stuff ...) ]
[(_ () seen lets bool (what x ... @ z) stuff ...)
(!-aux () seen lets bool (what x ... (reverse z)) stuff ...) ]
[(_ () ((a b c) z ...) lets 0 stuff ...)
(!-aux () ((a b c) z ...) lets (= 0 a) stuff ...) ]
[(_ () seen lets bool result stuff ...)
(let lets (do seen (bool result) stuff ...)) ]
))
(define-syntax !
(syntax-rules ()
[(_ specs bool stuff ...)
(!-aux specs () () bool stuff ...) ]
))
--
[T]he problem is that lispniks are as cultish as any other devout group and basically fall down frothing at the mouth if they see [heterodoxy].
--- Kenny Tilton
The good news is, it's not Lisp that sucks, but Common Lisp. --- Paul Graham --- Synchronet 3.21a-Linux NewsLink 1.2

From Newsgroup: comp.lang.lisp

CL-USER 1 > (loop for i for j in '(a b c) collect (cons i j))

((0 . A) (1 . B) (2 . C))

and, do you really like that code? is it readable?
if I encounter such stuff, i'd yell "WTF??".

I found a way around it 'for i from 0'

as others have noted, canonic way is to use upfrom. for i upfrom 0.
perfectly readable and makes sense for anybody, even if they do not know the language.

Let's see if we can make it shorter by using a Lispy language
instead of CL.

Gauche Scheme

(map cons (liota) '(a b c))
===>
((0 . a) (1 . b) (2 . c))
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From Newsgroup: comp.lang.lisp

B. Pym wrote:

Jochen wrote:

This ...

CL-USER 1 > (loop for i for j in '(a b c) collect (cons i j))

((0 . A) (1 . B) (2 . C))

works perfectly on LispWorks, but not with SBCL

FOR is an unknown keyword in FOR or AS clause in LOOP.

I found a way around it 'for i from 0' but am not sure if there is another smarter way to achieve this.

I think the canonical way would be

(loop for i upfrom 0
for j in '(a b c)
collect (cons i j))

newLISP

(map (curry list $idx) '(a b c))

((0 a) (1 b) (2 c))

Gauche Scheme

gosh> (map cons (liota) '(a b c))
((0 . a) (1 . b) (2 . c))

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:

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.

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!


John Foderaro:

I'm not trying to join a debate on loop. I just wanted to present
the other side of [the issue so that] the intelligent people can
then weigh the arguments on both sides.

I'm not suggesting that loop can be fixed either by adding
parenthesis or coming up with ways of indenting it to make it
understandable. It's a lost cause.

...

Another great example from kmp:

=== from kmp

For example, you might think
(loop with i = (random 100) for x from 1 to 10 do (print (list i x)))
and
(loop for i = (random 100) for x from 1 to 10 do (print (list i x)))
meant the same in English, [but they don't do the same thing in loop]

=== end kmp

loop lulls you into thinking that you understand the program since
you understand English. Make no mistake about it, loop is its
own language. If you use it you condem everyone who reads the
code to also learn the loop language.
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