[plt-scheme] Question about map

I think you are misusing letrec.

Try a let* instead.

-Felix

On Jul 3, 2008, at 4:38 PM, Marco Morazan wrote:

> 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)))
>    'ok))
>
> 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)
> ok
>>
>
> 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.
>
> Cheers,
>
> Marco
>
>
> 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 4.0.1.3-svn2jul2008 [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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