[plt-scheme] Question about map

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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