[racket-dev] Comparison functions and the `data' collection

From: Ryan Culpepper (ryan at cs.utah.edu)
Date: Thu Jun 21 18:04:59 EDT 2012

On 06/21/2012 09:38 AM, Eli Barzilay wrote:
> More than a week ago, Ryan Culpepper wrote:
>> On 06/11/2012 02:36 PM, Eli Barzilay wrote:
>>> Yesterday, Danny Yoo wrote:
>>>> It's a little unfortunate that there's a slight impedance mismatch
>>>> between what datum-order provides and what sort expects; the
>>>> my-less-than function in the example adapts the output of
>>>> datum-order so it can be used with sort.
>>> Thanks for pointing it out (I didn't know about it).  When I
>>> looked into this, I saw that in addition to `data/order' there is
>>> also an issue with `data/heap'.  It certainly does look like a
>>> problem worth addressing.
>>> The thing is that are two common ways to specify comparison functions:
>>> (a) three-valued comparison functions returning -1/0/+1, (b) a boolan
>>> strictly-smaller-than predicate.  The first is common in several
>>> mainsteam languages and the second is traditionally common in
>>> lisps (which leads to its use in the sort function).
>>> The issues are described very thoroughly in srfi-67 -- specifically,
>>> see the first two subsections in section 6.  I highly recommend
>>> reading that for this discussion.  They decide to go with option (a),
>>> with an explanation for the choice of a three-valued function and for
>>> their choice of -1/0/+1 values.
>> I don't remember if I discovered srfi-67 before or after I added
>> data/order.  In any case, I disagree with its rationale for -1/0/+1:
>> I dislike the idea of performing arithmetic on orderings.
> (See below.)
>> OTOH, data/order is used primarily by the ordered dictionary types
>> (data/splay-tree, data/skip-list), so it's probably missing operations
>> and conveniences for tasks like sorting a list.
> (I take it that they will be added, then...)
>>> 2. The `data/order' interface has several problems that I think is
>>>      best to resolve.
>>>      - Regadless of the above, it seems like a good idea to extend
>>>        the interface with a simple boolean predicate.  Maybe
>>>        something like `datum<?' and possibly others.  This would
>>>        address the issue that Danny raised above.
>>    From an order you can get the less-than and equality predicates:
>>     (order-<? datum-order)
>>     (order-=? datum-order)
> Yes, of course this is easy.  I can also do that `case' in a tiny λ.
> But it should be part of the library.
> On Saturday, Matthias Felleisen wrote:
>> This response leaves us with the impression that we whimsically
>> challenged established conventions in the general and the
>> Lisp-specific PL family. In addition it leaves us with
>> inconsistencies across our libraries, which I am coming to consider
>> more and more as a serious problem.
> Yes, that was exactly why I raised it.  Since it's core-ish
> functionality, having it behave well wrt expectations is IMO extremely
> important.  Right now we have a sad state of two different
> incompatibilities there, and I don't see any good explanation except
> for the "I disagree/dislike" (in contrast to the srfi).  Because of
> such expectations and because of existing code, I'd follow that
> without thinking about what might the right choice be.
> (Personally, I now think that the more lenient approach of any
> numbers, negative, 0, or positive is even better -- and I can also
> explain and justify why I think that, but that's irrelevant.)
>> Now -- I would be the last one to defend "established tradition"
>> over "getting things right" but at a minimum, the reasoning of why
>> we challenge tradition should be noted.
> +7

There's an advocate of my position in the SRFI discussion archives. See 
the following messages by Panu Kalliokoski:


Of course, the SRFI still went the other way. I would be happy to add 
functions to convert between the two conventions.


Posted on the dev mailing list.