[plt-scheme] Question about map

Yes, of course, Felix and Matthias. No claim made that my simplified
example reproducing the bug is beautiful. :-)

>
> I'd consider this a bug until someone convinces me that EoPL really wants
> rNrs silly letrec. -- Matthias
>

I would tend to agree. I assume it will be a while before this can be
addressed, no?

Cheers,

Marco

On Thu, Jul 3, 2008 at 5:04 PM, Matthias Felleisen <matthias at ccs.neu.edu> wrote:
>
>
> Felix is proposing a fix to your program logic. Because your program is
> using the bindings in a sequential manner, (let* ([fname ...] ...) ...) will
> do fine. In Scheme, letrec is the construct for mutually recursive bindings,
> and you're not declaring anything of this kind.
>
> I am conjecturing that, even though PLT Scheme's letrec promises a behavior
> for your letrec expression that is like the one for an equivalent let*
> expression, something in 4.0's expansion is failing this promise.
> (Technically, what is failing is a promise between the run-time system and
> your module. (I believe that) If we had pervasive contracts, we could issue
> a better error message, even if the error that you see is caused by an
> internal error.)
>
> -- Matthias
>
>
>
>
>
> On Jul 3, 2008, at 4:56 PM, Marco Morazan wrote:
>
>> How so?
>>
>> On Thu, Jul 3, 2008 at 4:45 PM, Felix Klock's PLT scheme proxy
>> <pltscheme at pnkfx.org> wrote:
>>>
>>> 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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