[plt-scheme] Question about map

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




On Jul 3, 2008, at 5:11 PM, Robby Findler wrote:

> Again, FWIW, looking at the source code suggests that eopl re-exports
> r5's letrec (which is supposed to do that, I believe) not #lang
> scheme's letrec.
>
> Robby
>
> On Thu, Jul 3, 2008 at 4:04 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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>>>>
>>>>
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>>