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

From: Robby Findler (robby at cs.uchicago.edu)
Date: Mon Jan 14 20:58:02 EST 2008

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  



On Jan 14, 2008, at 4:28 PM, John Clements <clements at brinckerhoff.org>  

> 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
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