[plt-scheme] Question about map

From: Marco Morazan (morazanm at gmail.com)
Date: Thu Jul 3 16:38:40 EDT 2008

Hi All,

I am a bit baffled. I have reproduced below a simplified example of my
rather large piece of software that produces the same error.

(define F '(define (f x) (+ x x)))

(define (all-but-last datum)
  (if (null? (cdr datum))
      (cons (car datum) (all-but-last (cdr datum)))))

(define (parse-test datum)
  (letrec ((fname (caadr datum))
           (params (cdadr datum))
           (localdefs (all-but-last (cddr datum)))
           (localdefs-names (map caadr localdefs)))

With the above I get:

Welcome to DrScheme, version 4.0.1tsrj2008-1 [3m].
Language: Essentials of Programming Languages (2nd ed.); memory limit:
128 megabytes.
> (all-but-last (cddr F))
> (map caadr (all-but-last (cddr F)))
> (parse-test F)
. . mcar: expects argument of type <mutable-pair>; given #<undefined>

As far as I can tell, (map caadr (all-but-last (cddr F))) and
(parse-test F) should yield the same result and (parse-test F) should
yield 'ok.

Now, here are the results with v372:

Welcome to DrScheme, version 372 [3m].
Language: Essentials of Programming Languages (2nd ed.).
> (all-but-last (cddr F))
> (map caadr (all-but-last (cddr F)))
> (parse-test F)

I think Eli is onto something. When I display the value of localdefs
in v4.0.1 it is #<undefined> while in v372 it is ().

I really hope I am missing something trivial and that I will have one
of those DUH! moments soon. In the meantime, I am really baffled.

Once again, any pointers would be appreciated.



On Thu, Jul 3, 2008 at 3:45 PM, Eli Barzilay <eli at barzilay.org> wrote:
> On Jul  3, Robby Findler wrote:
>> AFAICT, EoPL works fine with the basic example you're giving. I'm
>> still not able to divine what is going wrong:
>> Welcome to DrScheme, version [3m].
>> Language: Essentials of Programming Languages (2nd ed.); memory limit:
>> 128 megabytes.
>> > (map caadr (list '(lambda (x) x)))
>> (x)
> My guess is that there are two problems here: the first is that `map'
> is bound to `mmap', and `caadr' is bound to a function that composes
> `mcar' and `mcdr' -- and these make error messages confusing Marco
> here:
>> >>> My code does not use mcar anywhere. I must assume it has to do
>> >>> with the implementation of map (?).
> But the real problem is elsewhere:
>> >>> This code: (map caadr localdefs)
>> >>>
>> >>> Produces this error:
>> >>>
>> >>> mcar: expects argument of type <mutable-pair>; given
>> >>> #<undefined>
> You must have some `undefined' value somewhere, and that's probably
> due something that changed in the language.  (And because of the above
> the error message doesn't help.)
> To find the bad value I'd start with
>  (map (lambda (x)
>         (if (and (pair? x) (pair? (cdr x)) (pair? (cadr x)))
>           (caadr x)
>           (error 'blah "~s" x)))
>       localdefs)
> and go on from there.
> --
>          ((lambda (x) (x x)) (lambda (x) (x x)))          Eli Barzilay:
>                  http://www.barzilay.org/                 Maze is Life!

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