[plt-scheme] caution about use of 'pair?' in programs with mutable pairs?

From: John Clements (clements at brinckerhoff.org)
Date: Mon Jan 14 23:38:36 EST 2008

On Jan 14, 2008, at 5:58 PM, Robby Findler wrote:

> This is the wrong default thing to try. First try avoiding mutation  
> (see my blog post for a time when I didn't follow this advice and  
> suffered)

Absolutely; this goes without question.


> Robby
> Robby
> On Jan 14, 2008, at 4:28 PM, John Clements  
> <clements at brinckerhoff.org> wrote:
>> On Jan 14, 2008, at 4:13 PM, Matthias Felleisen wrote:
>>> On Jan 14, 2008, at 3:06 PM, John Clements wrote:
>>>> I feel lucky not to have been bitten by this, but I can see some  
>>>> potentially very painful bugs possible in a switchover from v3  
>>>> code to v4 code; in particular, code that uses 'pair?' to  
>>>> distinguish empty from nonempty lists could misclassify mpairs  
>>>> as nulls, leading to some very unpleasant "non-fail-fast"  
>>>> errors.  Instead, it would seem to be wiser here to use (not  
>>>> (null? ...)) rather than (pair? ...)
>>> I don't understand your remark. When you know pair? of some value  
>>> v, you can use car/cdr. If you know (not (null? v)), what do you  
>>> really know?
>> The scenario I'm imagining is this: I have an existing piece of  
>> code that is written using mutable pairs.  I upgrade to v4, and it  
>> stops working.  I get it working again by replacing instances of  
>> set-car! and set-cdr! with set-mcar! and set-mcdr!.  Along the  
>> way, I discover places where I need to construct lists using mlist  
>> and mcons rather than list and cons.  Now imagine that somewhere  
>> in there is this piece of code:
>> (define (my-length l)
>>  (cond [(pair? l) (+ 1 (my-length (mcdr l)))]
>>        [else 0]))
>> This will return zero on any mlist.  This is why I'm worried about  
>> uses of 'pair?'; because unlike mcar & mcdr, they don't  
>> immediately report an error when called with the wrong flavor of  
>> list.
>> Again, this is all FWIW; perhaps no one will actually run into  
>> this bug, or perhaps this is a tiny instance in a much larger pool  
>> of similar possible scheme bugs.
>> John
>> _________________________________________________
>> For list-related administrative tasks:
>> http://list.cs.brown.edu/mailman/listinfo/plt-scheme

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