Re: Two maybe-bugs and two proposals

[Sebastian Egner <sebastian.egner@xxxxxxxxxxx>]

Hello Amir,

Thank you for the remarks.

> Actually, it would work iff thereone exactly
one class.

You are right; we'll correct it.

> Also, in an impure language such is Scheme, a
comparison function
> can be defined from equality - but not in O(1)
operations.

We state that already by saying there
are no "sublinear worst-case" data structures
for this. I'll rewrite the remark to
make it clearer and given an implementation explicitly;
it might prove useful to somebody someday.

> The specification of the chain functions (#node_idx_90)
states clearly 
> that when exactly one value is given, the comparison function should
> still be invoked. The purpose of this is not clear, since the only
> difference between this comparison and a no-op is type-checking. IMO,
> there is no use for this kind of uniformity here (to me it doesn't
seem 
> uniform at all).

The purpose of calling the compare procedure
for the only element
of  a chain of length one is indeed
type-checking: I am not sure what
you mean by "uniformity" here,
but after calling

        (apply
chain<? compare-integer xs)

you may assume that xs is a list of
objects satisfying integer?,
irrespectively of the length of xs or
the outcome of the test.
(We'll add a clarifying remark in the
specification.)

In my experience, most people will assume
that xs is a list of
integers anyhow, no matter what is actually
specified for chain<?.
(And we try to minimize the "gotcha
potential.")

> I would like the <? family of functions to
take only one parameter, the 
> comparison function, so that they will transform from comparison 
> functions to predicates. This is also doable with srfi-26 as (cut
<? <> 
> <>), but for many uses, including the second proposal, the shorter
form 
> is nicer.
>
> Another proposal is to drop the `chain' functions. There is one basic
> higher order list function, (chain pred . elts) that applies the elts
in 
> consecutive pairs. If the previous proposal is also accepted, all
that 
> would be required from uses is typing (chain (<? comp) ...) instead
of 
> (chain<? comp ...). It would also allow non-srfi-67 uses of this
list 
> function, and using default-compare: (chain <? ...) works as (chain<?
> default-compare ...) does in the document.

Incidentally, we had this design in
an earlier (before release) version
of the SRFI and we switched to the current
design later.
However, we switched before we had invented
IF<? and friends, which
were conceived when I caught myself
avoiding (if ((<? compare) x y) ..) in
the reference implementation for minor
efficiency concerns and lying to
myself about efficiency in the spirit
of "but in the application I will use it."
Now may be the time to revise the earlier
decision, as you propose.

The alternatives are: Parametric tests
(<? compare x y) vs. higher-order
procedures (<? compare) => predicate.


I would like to think about this first,
and come back to it later.

For the chain tests: The five tests
currently specified make sense and are
clearly defined and efficient. (chain?
(not=? compare) x ...) is very dangerous,
and the need for non-SRFI-67 use of
chain testing is hardly an issue. So,
at least I am pretty happy with chain<?
and friends and pairwise-not=?.

Sebastian.

P.S. Sorry for copying you email once
again below, but Philips uses Lotus Notes....
----
Dr. Sebastian Egner
Senior Scientist Channel Coding & Modulation
Philips Research Laboratories
Prof. Holstlaan 4 (WDC 1-051, 1st floor, room 51)
5656 AA Eindhoven
The Netherlands
tel:       +31 40 27-43166   *** SINCE 10-Feb-2005
***
fax:      +31 40 27-44004
email: sebastian.egner@xxxxxxxxxxx

srfi-67-request@xxxxxxxxxxxxxxxxx 14-04-2005 13:57

To:        srfi-67@xxxxxxxxxxxxxxxxx         cc:        (bcc: Sebastian Egner/EHV/RESEARCH/PHILIPS)         Subject:        Two maybe-bugs and two proposals         Classification:

In the question about deriving ordering from equality
(#node_toc_node_sec_Temp_25), it is claimed that the definition doesn't
work if there is more than one element. Actually, it would work iff 
thereone exactly one class. Also, in an impure language such is Scheme,
a comparison function can be defined from equality - but not in O(1) 
operations.

The specification of the chain functions (#node_idx_90) states clearly
that when exactly one value is given, the comparison function should 
still be invoked. The purpose of this is not clear, since the only 
difference between this comparison and a no-op is type-checking. IMO, 
there is no use for this kind of uniformity here (to me it doesn't seem
uniform at all).

I would like the <? family of functions to take only one parameter,
the 
comparison function, so that they will transform from comparison 
functions to predicates. This is also doable with srfi-26 as (cut <?
<> 
<>), but for many uses, including the second proposal, the shorter
form 
is nicer.

Another proposal is to drop the `chain' functions. There is one basic 
higher order list function, (chain pred . elts) that applies the elts in
consecutive pairs. If the previous proposal is also accepted, all that
would be required from uses is typing (chain (<? comp) ...) instead
of 
(chain<? comp ...). It would also allow non-srfi-67 uses of this list
function, and using default-compare: (chain <? ...) works as (chain<?
default-compare ...) does in the document.