re: ELSE in LOOP

[WJ]

Michael Kappert wrote:

> As a matter of style, i suggest using IF instead of WHEN
> if (err, when :-)) the ELSE branch is present.
>
> So, your code would appear as
>
> (loop for i from 0 to 10
> if (evenp i)
> do (print i)
> and if (= i 6)
> do (print 'BINGO!!)
> else do ()
> else
> do (print 'odd))
>
> which gives
>
> 0
> odd
> 2
> odd
> 4
> odd
> 6
> bingo!!
> odd
> 8
> odd
> 10
> odd
> nil

Wrong. In SBCL:

* (loop for i from 0 to 10
if (evenp i)
do (print i)
and if (= i 6)
do (print 'BINGO!!)
else do ()
else
do (print 'odd))

debugger invoked on a SB-INT:SIMPLE-PROGRAM-ERROR:
A compound form was expected, but NIL found.
current LOOP context: DO NIL ELSE.


Racket:

(for ([i 11])
(if (even? i)
(begin
(displayln i)
(when (= 6 i) (displayln 'Bingo!)))
(displayln 'odd)))

==>
0
odd
2
odd
4
odd
6
Bingo!
odd
8
odd
10

[WJ]

Barry Margolin wrote:

> >(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.

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.


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.

[WJ]

WJ wrote:

>
> Racket:
>
> (for ([i 11])
> (if (even? i)
> (begin
> (displayln i)
> (when (= 6 i) (displayln 'Bingo!)))
> (displayln 'odd)))

Shorter:

(for ([i 11])

(displayln (if (even? i) i 'odd))

[kodifik]

The correct form is :
(loop for i from 1 to 10 do
(if (evenp i)
(if (= (print i) 6)
(print 'BINGO!!))
(print 'odd)))

[kodifik]

On 23 mayo, 10:12, "WJ" <w_a_x_...@yahoo.com> wrote:
> Wrong.  In SBCL:
>
> * (loop for i from 0 to 10
>     if (evenp i)
>       do (print i)
>       and if (= i 6)
>             do (print 'BINGO!!)
>           else do ()
>     else
>       do (print 'odd))

Also wrong. Correct :

[WJ]

Gauche Scheme:

(do-ec (: i 11)
(if (even? i)
(begin
(print i)
(when (= 6 i) (print 'Bingo!)))
(print 'odd)))

===>

[WJ]

In ABCL:

Debugger invoked on condition of type PROGRAM-ERROR:

A compound form was expected, but NIL found.

Current LOOP context: DO NIL ELSE.


MatzLisp (Ruby):

11.times{|i|
if i.even?
puts i
puts 'Bingo!' if 6==i
else
puts :odd
end}

===>
0
odd
2
odd
4
odd
6
Bingo!
odd
8
odd
10

--
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at Boston University admitted that, "There is no question but that Dr. King
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